[Update June 20 2009
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.
The final bill is linked to this post. The details below refer to an older bill.]
Cash for Clunkers is back thanks to Rep. Betty Sutton of Ohio. See this April 1 post for useful tables showing the gas and cost savings you would receive under the new plan; for a recap of the earlier 2009 versions, see this post. See the blog archive on the right for older and newer posts on Cash for Clunkers.
As we argued in our original post on Cash for Clunkers, encouraging someone to replace a 14 MPG car with a 25 MPG car saves 300 gallons of gas per 10,000 miles of driving, or 3 tons of carbon dioxide. There is no improvement that one can make to a 33 MPG car that will reduce carbon dioxide by the same amount. GPM makes the greenhouse gas benefits of Cash for Clunkers clear.
The Sutton Bill
Rep. Betty Sutton of Ohio has introduced a new bill that offers up to $5,000 towards purchasing a new, US-made car that gets 27 MPG (or 24 MPG for a truck or SUV), and up to $4,000 towards purchasing a Canadian or Mexican made car that gets 30 MPG. Cars assembled outside the US can be eligible but cars manufactured outside the US are not. The new car must cost $35,000 or less.
This bill requires that the trade-in car must be at least 8 years old and must be junked.
Autobloggreen reports that the payment for the new car is graduated and increases with the new cars efficiency. Details, however, are hard to come by.
Whereas the original bill focused on removing the most inefficient cars (below 18 MPG, or 5.5 gallons per 100 miles) and replacing them with efficient cars (25% better MPG than the class average, around 4 gallons per 100 miles), this bill appears to allow a car buyer to turn in any 8+ year old car as long as the new car is more efficient and above the stated minimum MPG for the class. That makes more cars eligible for the trade in. It also means that some trade ins may have only a small impact on CO2 reductions (e.g., should someone trade in a 25 MPG car for a 28 MPG car).
See also coverage at the Detroit Free Press and at Kicking Tires.
Using behavioral insights to promote smarter decisions about energy efficiency and energy use.
Sunday, March 22, 2009
Saturday, March 14, 2009
Overview of GPM - Updated
Know Your GPM
Know your gas consumption and compare it to any 2009 car here.
Overview
Here is a quick overview of what's wrong with MPG and how "Gallons per mile" (GPM) helps:
MPG tricks people's perceptions. Replacing a car that gets 14 MPG with a car that gets 17 MPG saves as much gas for a given distance as replacing a car that gets 33 MPG with a car that gets 50 MPG (about 1 gallon per hundred miles--see this table). MPG obscures the value of removing the most inefficient cars. As the GPM table shows, a 14 to 20 MPG improvement saves twice as much gas as a 33 to 50 MPG improvement:
"Gallons per 100 miles" or "Gallons per 10,000 miles" (GPM) corrects these misperceptions. These gas consumption measures should be provided by consumer sites to supplement information about MPG. And GPM makes clear that policy should be focused on replacing the most inefficient cars. [Update July 2009: We applaud the USA Today and Popular Mechanics for adding "gallons per 100 miles" to their car reviews.]
A free link to the original Science article can be found here and a post summarizing the original Science studies can be found here.
A GPM calculator can be found here. It allows you to calculate gas consumption (and costs) for your current car and for all 2009 cars. This is a model of what the EPA and Consumer Reports should offer to fix the illusions caused by MPG. Offering a gas consumption measure (GPM) in addition to MPG is a low cost way to give consumers better information - as proposed by Representative James Sensenbrenner. It should be a bipartisan no-brainer.
As we describe below, both gas savings and CO2 reductions are a linear function of decreases in GPM; neither is a linear function of increases in MPG.
A more detailed post summarizing the basic argument for GPM can be found here. The end of this longer post explains why "going metric" doesn't solve the MPG illusion and why "percentage improvement" also leads to misperceptions.
The Math
The math is trivial. GPM is the inverse of MPG, and the relationship is curvilinear. What is not trivial is that car buyers assume that they can take a difference in MPG when comparing cars to gauge gas savings. GPM, but not MPG, operates by subtraction.
The math to compare fuel economy across cars is not trivial. It involves more than taking the simple inverse of GPM = 1/MPG. Specifically, the math requires taking some distance, X, and dividing it by two MPG figures before taking a difference: X/MPGhigh - X/MPGlow. For example, the improvements from 10 to 11 MPG, 16.5 to 20 MPG, and 33 to 50 MPG all save the same amount of gas over a given distance (e.g., 100 gallons per 10,000 miles). To measure gas savings, MPG requires division before subtraction (e.g., 10000/20 - 10000/16.5).
However, people intuitively rely on subtraction when comparing MPG, which creates illusions.
GPM, not MPG, allows car buyers to use subtraction to compare the fuel economy of different cars (e.g., 600 vs. 500 gallons per 10,000 miles). GPM makes the magnitude of gas savings clear without additional math.
Europe, Canada, and Australia use volume over distance (liters per 100 kilometers); India and Japan, like the US, use distance over volume (kilometers over liters). Kilometers per liter creates the same illusions as MPG.
Summary: The Case for GPM
Which is more useful to know: How far you can drive on a gallon of gas? Or, how much gas you will use while owning a car?
MPG answers the first question. It is useful when judging the range of one's gas tank (can I make it two more exits before a refill?). But it answers a less important question. GPM answers the question of gas consumption. We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions (100 gallons of gasoline = 1 ton of CO2). MPG cannot.
Differences in GPM provide a direct measure of gas savings and CO2 reductions.
Differences in MPG do not.
Providing a column of GPM numbers at Consumer Reports and at fueleconomy.gov would make accurate fuel economy comparisons far easier than the current column of MPG numbers. GPM needs to supplement MPG as a measure of fuel economy.
Know your gas consumption and compare it to any 2009 car here.
Overview
Here is a quick overview of what's wrong with MPG and how "Gallons per mile" (GPM) helps:
MPG tricks people's perceptions. Replacing a car that gets 14 MPG with a car that gets 17 MPG saves as much gas for a given distance as replacing a car that gets 33 MPG with a car that gets 50 MPG (about 1 gallon per hundred miles--see this table). MPG obscures the value of removing the most inefficient cars. As the GPM table shows, a 14 to 20 MPG improvement saves twice as much gas as a 33 to 50 MPG improvement:
| MPG | GPHM |
|---|---|
| 10 | 10 |
| 11 | 9 |
| 12.5 | 8 |
| 14 | 7 |
| 16.5 | 6 |
| 20 | 5 |
| 25 | 4 |
| 33 | 3 |
| 50 | 2 |
| 100 | 1 |
"Gallons per 100 miles" or "Gallons per 10,000 miles" (GPM) corrects these misperceptions. These gas consumption measures should be provided by consumer sites to supplement information about MPG. And GPM makes clear that policy should be focused on replacing the most inefficient cars. [Update July 2009: We applaud the USA Today and Popular Mechanics for adding "gallons per 100 miles" to their car reviews.]
A free link to the original Science article can be found here and a post summarizing the original Science studies can be found here.
A GPM calculator can be found here. It allows you to calculate gas consumption (and costs) for your current car and for all 2009 cars. This is a model of what the EPA and Consumer Reports should offer to fix the illusions caused by MPG. Offering a gas consumption measure (GPM) in addition to MPG is a low cost way to give consumers better information - as proposed by Representative James Sensenbrenner. It should be a bipartisan no-brainer.
As we describe below, both gas savings and CO2 reductions are a linear function of decreases in GPM; neither is a linear function of increases in MPG.
A more detailed post summarizing the basic argument for GPM can be found here. The end of this longer post explains why "going metric" doesn't solve the MPG illusion and why "percentage improvement" also leads to misperceptions.
The Math
The math is trivial. GPM is the inverse of MPG, and the relationship is curvilinear. What is not trivial is that car buyers assume that they can take a difference in MPG when comparing cars to gauge gas savings. GPM, but not MPG, operates by subtraction.
The math to compare fuel economy across cars is not trivial. It involves more than taking the simple inverse of GPM = 1/MPG. Specifically, the math requires taking some distance, X, and dividing it by two MPG figures before taking a difference: X/MPGhigh - X/MPGlow. For example, the improvements from 10 to 11 MPG, 16.5 to 20 MPG, and 33 to 50 MPG all save the same amount of gas over a given distance (e.g., 100 gallons per 10,000 miles). To measure gas savings, MPG requires division before subtraction (e.g., 10000/20 - 10000/16.5).
However, people intuitively rely on subtraction when comparing MPG, which creates illusions.
GPM, not MPG, allows car buyers to use subtraction to compare the fuel economy of different cars (e.g., 600 vs. 500 gallons per 10,000 miles). GPM makes the magnitude of gas savings clear without additional math.
Europe, Canada, and Australia use volume over distance (liters per 100 kilometers); India and Japan, like the US, use distance over volume (kilometers over liters). Kilometers per liter creates the same illusions as MPG.
Summary: The Case for GPM
Which is more useful to know: How far you can drive on a gallon of gas? Or, how much gas you will use while owning a car?
MPG answers the first question. It is useful when judging the range of one's gas tank (can I make it two more exits before a refill?). But it answers a less important question. GPM answers the question of gas consumption. We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions (100 gallons of gasoline = 1 ton of CO2). MPG cannot.
Differences in GPM provide a direct measure of gas savings and CO2 reductions.
Differences in MPG do not.
Providing a column of GPM numbers at Consumer Reports and at fueleconomy.gov would make accurate fuel economy comparisons far easier than the current column of MPG numbers. GPM needs to supplement MPG as a measure of fuel economy.
An Open Letter to the EPA and the Department of Energy
March 14, 2009
Office of the Administrator
Environmental Protection Agency
Ariel Rios Building
1200 Pennsylvania Ave., NW
Washington, DC 20004
Dr. Stephen Chu
Secretary of Energy
U. S. Department of Energy
1000 Independence Ave., SW
Washington, DC 20585
Dear Administrator Jackson and Secretary Chu,
I am writing to ask you to consider adding gas consumption information to the fuel efficiency information currently provided on automobiles sold in the United States. Currently, as required by law, the EPA provides information on “miles per gallon” (MPG). However, in research published in June, 2008 in Science (Larrick and Soll, "The MPG Illusion," Science, 320 (5883) p. 1593-1594), my co-author and I showed that MPG misleads car buyers when they compare the gas savings available from different cars.
The following table shows gas consumption for vehicles with different levels of MPG over 10,000 miles of driving:
10.0 MPG = 1000 GPM (Gallons per 10,000 Miles)
11.0 MPG = 900 GPM
12.5 MPG = 800 GPM
14.0 MPG = 700 GPM
16.5 MPG = 600 GPM
20.0 MPG = 500 GPM
25.0 MPG = 400 GPM
33.0 MPG = 300 GPM
50.0 MPG = 200 GPM
Because GPM is the inverse of MPG, gas savings declines rapidly as MPG increases. Thus, the largest gas savings come from removing the most inefficient vehicles. Replacing a car that gets 14 MPG with a car that gets 25 MPG reduces gas consumption (and carbon dioxide emissions) more than any possible replacement of a 33 MPG car over the same distance. MPG obscures these savings.
The research in the Science article showed that people tend to subtract MPG when they assess the gas savings of different vehicles. This “linear” thinking creates illusions, leading people to undervalue replacing inefficient cars. Only a measure of GPM can be subtracted.
What can the EPA and the Energy Department do to help consumers? The remainder of this letter suggests possible improvements to the fueleconomy.gov website and the annual Fuel Economy Guide (http://www.fueleconomy.gov/feg/FEG2009.pdf) which are jointly run by the EPA and the Office of Energy Efficiency and Renewable Energy (EERE) of the Department of Energy.
First, there are two things that the EPA and DOE do not need to do. First, you do not need to explain the MPG Illusion to car buyers, which may not be of interest to many consumers. Second, you do not need to use the acronym “GPM,” which may strike some consumers as unnecessary.
What the EPA and EERE should consider doing is the following:
1) Emphasize clearly on the opening page of the fueleconomy.gov website (and in the Fuel Economy Guide) that consumers should compare the gas consumption of cars, not MPG. Almost all consumers will care about gas consumption. It needs no further justification.
2) Provide consumers with a salient, immediate measure of gas consumption when they examine cars. The measure could be gallons per 100 miles or gallons per 10,000 miles. It could be displayed as a column of data in the Fuel Economy Guide and could be exhibited next to MPG in the car comparison windows on the fueleconomy.gov.
Having the option of tailoring the distance to a consumer’s choice is also useful. The following Duke Calculator is a model for this type of information. It allows car buyers to calculate gas consumption for their current vehicle (calculator 1) and compare it to all new 2009 vehicles (calculators 2 and 3):
The current fueleconomy.gov website provides some of this information, but only to a limited extent.
There is an option of switching to gallons per 100 miles—although no explanation is given for why consumers would want to focus on gallons per 100 miles. Because the fueleconomy.gov site never mentions the need to compare gas consumption, few consumers are likely to elect the option.
The fueleconomy.gov website automatically displays fuel consumption per 25 miles for each vehicle. However, the distance is so small that it makes all cars appear very similar in terms of gas consumption. Using a larger base—at least 100 miles, or better, 1,000 miles or 10,000 miles—makes differences in gas consumption between cars clear.
The site also automatically displays a number of annual measures that are linear proxies of gas consumption: Barrels of oil consumed, carbon footprint, and gas costs. These proxies are very useful. However, none of these are effective as a standard, memorable unit of efficiency (i.e., they are not comparable to MPG in terms of how well they "stick"): Barrels and carbon footprint are unfamiliar. Gas costs fluctuate with gas prices so memorizing listed gas costs in the year of purchase (if possible) is uninformative for later car purchase decisions. In contrast to the proxies, gas consumption is intuitive. Like MPG, a standard measure of gas consumption can be remembered and compared across cars and over time.
In sum, you will be helping consumers to make wiser decisions about gas consumption, and carbon dioxide emissions, by providing them with salient information on gas consumption. Please emphasize and make easily available information on gas consumption.
Sincerely,
Richard Larrick
larrick@duke.edu
Updated: 2:40 pm March 16
Sunday, February 8, 2009
GM offers a GPM calculator
[Update June 20 2009
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.
The final bill is linked to this post. The details below refer to an older bill.]
How does MPG translate to gas use and gas cost? How do different 2009 cars compare? See our GPM ("gallons per mile") calculator here: http://www.gpmcalculator.com.
The General Motors website offers a kind of GPM calculator that allows you to compare GM vehicles on MPG, carbon emissions, gas cost, etc. It can be tailored to your choice of driving distance, driving mix (city/highway), and expected gas prices.
In a very interesting design choice, GM describes gas consumption not as gallons of gasoline, but as "barrels of fuel." I assume this is barrels of oil.
In a new article (Burson, Larrick, & Lynch, 2009, Psychological Science), my co-authors and I have shown that people are more sensitive to differences expressed on "expanded" scales (e.g., 48 vs. 36 inches) than on "contracted" scales (e.g., 4 vs 3 feet). In this case, "gallons" is an expanded scale (and the one we're used to), while "barrels" is a contracted (and unfamiliar) scale. Why use a "contracted" scale such as barrels?
General Motors' stock ticker abbreviation is GPM.
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.
The final bill is linked to this post. The details below refer to an older bill.]
How does MPG translate to gas use and gas cost? How do different 2009 cars compare? See our GPM ("gallons per mile") calculator here: http://www.gpmcalculator.com.
The General Motors website offers a kind of GPM calculator that allows you to compare GM vehicles on MPG, carbon emissions, gas cost, etc. It can be tailored to your choice of driving distance, driving mix (city/highway), and expected gas prices.
In a very interesting design choice, GM describes gas consumption not as gallons of gasoline, but as "barrels of fuel." I assume this is barrels of oil.
In a new article (Burson, Larrick, & Lynch, 2009, Psychological Science), my co-authors and I have shown that people are more sensitive to differences expressed on "expanded" scales (e.g., 48 vs. 36 inches) than on "contracted" scales (e.g., 4 vs 3 feet). In this case, "gallons" is an expanded scale (and the one we're used to), while "barrels" is a contracted (and unfamiliar) scale. Why use a "contracted" scale such as barrels?
General Motors' stock ticker abbreviation is GPM.
Saturday, February 7, 2009
Cash for Clunkers 4
[Update June 20 2009
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than the voucher.
The final bill is linked to this post. The details below refer to an older bill.]
A Stabenow and Harkin Cash for Clunkers bill, which replaced the original Feinstein et al proposal, has been dropped from the economic stimulus package. It is effectively moribund. See the Detroit Free Press summary here.
The latest Cash for Clunkers bill had a stronger focus on helping US auto manufacturers. Here is a brief description from the Free Press article:
"The proposal would have required that any new vehicles bought with the money be built in the United States, which foreign automakers said today would violate international trade agreements.
Under the plan crafted by Michigan Sen. Debbie Stabenow, D-Mich., and Sen. Tom Harkin, D-Iowa, any vehicle 10 years old or younger could be traded in for $10,000. Owners would have to have family income of less than $75,000 a year, own fewer than three vehicles and have owned the vehicle they’re trading in before Jan. 16.
Buyers would have to choose from new cars that get at least 25 miles per gallon and trucks that get 20 m.p.g., and the new model would have to get at least 5 m.p.g. better than the old vehicle. The trade-in would have to be crushed. "
There are some attractive features to the latest Stabenow and Harkin proposal. Requiring that the cars are 10 years or younger helps reduce the junker moral hazard problem. (But sources are confusing on this point: This Free Press article states that the cars need to be at least 10 years old.) And offering up to $10,000 would help pull in highly inefficient cars that still have a high trade in value (e.g., a 2004 Yukon or Tahoe).
Of course, limiting the offer to households with an income of $75,000 or less cuts out many owners of expensive inefficient cars. I'm always amazed at these stark threshhold effects in the law--going from an income of $74,999 to $75,001 drops the credit from $10,000 to $0. Why can't it be more finely graduated than that?
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than the voucher.
The final bill is linked to this post. The details below refer to an older bill.]
A Stabenow and Harkin Cash for Clunkers bill, which replaced the original Feinstein et al proposal, has been dropped from the economic stimulus package. It is effectively moribund. See the Detroit Free Press summary here.
The latest Cash for Clunkers bill had a stronger focus on helping US auto manufacturers. Here is a brief description from the Free Press article:
"The proposal would have required that any new vehicles bought with the money be built in the United States, which foreign automakers said today would violate international trade agreements.
Under the plan crafted by Michigan Sen. Debbie Stabenow, D-Mich., and Sen. Tom Harkin, D-Iowa, any vehicle 10 years old or younger could be traded in for $10,000. Owners would have to have family income of less than $75,000 a year, own fewer than three vehicles and have owned the vehicle they’re trading in before Jan. 16.
Buyers would have to choose from new cars that get at least 25 miles per gallon and trucks that get 20 m.p.g., and the new model would have to get at least 5 m.p.g. better than the old vehicle. The trade-in would have to be crushed. "
There are some attractive features to the latest Stabenow and Harkin proposal. Requiring that the cars are 10 years or younger helps reduce the junker moral hazard problem. (But sources are confusing on this point: This Free Press article states that the cars need to be at least 10 years old.) And offering up to $10,000 would help pull in highly inefficient cars that still have a high trade in value (e.g., a 2004 Yukon or Tahoe).
Of course, limiting the offer to households with an income of $75,000 or less cuts out many owners of expensive inefficient cars. I'm always amazed at these stark threshhold effects in the law--going from an income of $74,999 to $75,001 drops the credit from $10,000 to $0. Why can't it be more finely graduated than that?
Cash for Clunkers 3
[Update June 20 2009
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than the voucher.
The final bill is linked to this post. The details below refer to an older bill.]
The cash for clunkers proposal raises two moral hazard questions. One is whether people will fix up junked cars to trade in for the cash; if so, this may help the economy, but does little for reducing greenhouse gas emissions. The current Senate plan tries to minimize this effect by requiring that a car be registered for 120 days before being turned in.
The second moral hazard question is the degree to which improvements in fuel efficiency lead people to drive more (because driving is now less costly). If this "rebound effect" is large, it could wipe out most of the greenhouse gas benefit of replacing a clunker with a more efficient car. The MIT Technology Review reports:
"Some experts counter that the rebound effect is small. For one thing, there is a limit to how much more people will drive, regardless of how little they pay for gas. (Several estimates, based on data from past fuel-economy standards, suggest that for every 10 percent improvement in fuel economy, about 20 percent of the improvement, or two percentage points, is lost because people drive more.)"
The implication is that the actual carbon reduction will be close to the amount projected. For example, the move from 14 MPG to 25 MPG is projected to eliminate 3 tons of carbon dioxide over 10,000 miles; in effect, the replacement will tend to eliminate 2.4 tons of CO2 because the car owner will drive the new 25 MPG car more than 10,000 miles. That's still a big benefit in terms of GHG reduction.
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than the voucher.
The final bill is linked to this post. The details below refer to an older bill.]
The cash for clunkers proposal raises two moral hazard questions. One is whether people will fix up junked cars to trade in for the cash; if so, this may help the economy, but does little for reducing greenhouse gas emissions. The current Senate plan tries to minimize this effect by requiring that a car be registered for 120 days before being turned in.
The second moral hazard question is the degree to which improvements in fuel efficiency lead people to drive more (because driving is now less costly). If this "rebound effect" is large, it could wipe out most of the greenhouse gas benefit of replacing a clunker with a more efficient car. The MIT Technology Review reports:
"Some experts counter that the rebound effect is small. For one thing, there is a limit to how much more people will drive, regardless of how little they pay for gas. (Several estimates, based on data from past fuel-economy standards, suggest that for every 10 percent improvement in fuel economy, about 20 percent of the improvement, or two percentage points, is lost because people drive more.)"
The implication is that the actual carbon reduction will be close to the amount projected. For example, the move from 14 MPG to 25 MPG is projected to eliminate 3 tons of carbon dioxide over 10,000 miles; in effect, the replacement will tend to eliminate 2.4 tons of CO2 because the car owner will drive the new 25 MPG car more than 10,000 miles. That's still a big benefit in terms of GHG reduction.
Wednesday, February 4, 2009
NSF and Truck Trend Magazine
The National Science Foundation discusses the MPG Illusion in a report called "Math: What's the Problem?" and Truck Trend magazine discusses it in an article on fuel efficient trucks.
Saturday, January 31, 2009
A letter from Rep. James Sensenbrenner to EPA Aministrator Lisa Jackson
Representative James Sensenbrenner of Wisconsin has sent a letter to new EPA Administrator Lisa Jackson calling attention to the benefits of using gallons per mile as measure of fuel economy. Read the press release and letter here.
Rep. Sensenbrenner has also proposed support for hybrid trucks. Given the low MPG of trucks (delivery trucks, garbage trucks, etc.), we believe this makes a great deal of sense. GPM makes clear that even a modest MPG improvement on trucks will save tons of carbon over 10,000 miles of driving.
Quoting from the press release: "In reducing fuel consumption, our focus should start with the least efficient vehicles. Because trucks consume much greater quantities of fuel than cars, even seemingly modest efficiency gains result in substantial fuel savings," Sensenbrenner said. "Trucks consume 48 percent of our fuel and each individual truck consumes substantially more fuel than a passenger car. Trucks, not cars, are the low-hanging fruit, and trucks should be a primary focus of hybrid technology research."
The press release then describes the benefits of replacing conventional garbage trucks with hybrid garbage trucks. We think this is a compelling application of the GPM argument.
Coca Cola recently purchased 185 hybrid delivery trucks.
Rep. Sensenbrenner has also proposed support for hybrid trucks. Given the low MPG of trucks (delivery trucks, garbage trucks, etc.), we believe this makes a great deal of sense. GPM makes clear that even a modest MPG improvement on trucks will save tons of carbon over 10,000 miles of driving.
Quoting from the press release: "In reducing fuel consumption, our focus should start with the least efficient vehicles. Because trucks consume much greater quantities of fuel than cars, even seemingly modest efficiency gains result in substantial fuel savings," Sensenbrenner said. "Trucks consume 48 percent of our fuel and each individual truck consumes substantially more fuel than a passenger car. Trucks, not cars, are the low-hanging fruit, and trucks should be a primary focus of hybrid technology research."
The press release then describes the benefits of replacing conventional garbage trucks with hybrid garbage trucks. We think this is a compelling application of the GPM argument.
Coca Cola recently purchased 185 hybrid delivery trucks.
Friday, January 30, 2009
Cash for Clunkers 2
[Update June 20 2009
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.
The final bill is linked to this post. The details below refer to an older bill.]
Following up on this earlier post, I have tried to find out how much carbon dioxide is emitted in the process of manufacturing new cars. If this amount is substantial, it calls into question the GHG benefits of replacing inefficient cars with more efficient cars.
Honda reports that it emits less than a ton of carbon dioxide producing each vehicle. To put this number in perspective, a 20 MPG car will emit 50 tons of CO2 over 100,000 miles; a 30 MPG car will emit 33 tons of CO2. The ton of CO2 emitted when producing a new car is a trivial proportion of the CO2 emitted when driving.
Consistent with Honda's claim, this WRI report says that assembly accounts for 2% of total carbon emissions from cars.
The implication: The carbon emitted to produce a new car is not a compelling reason to oppose the Cash for Clunkers plan. Gas savings will make up for the manufacturing emissions in less than a year's worth of driving.
(May 7 Update: Bill Chameides, Dean of Duke's Nicholas School of the Environment, says that CO2 from manufacture is closer to 7 tons of CO2.)
See the blog archive on the right for newer posts on Cash for Clunkers.
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.
The final bill is linked to this post. The details below refer to an older bill.]
Following up on this earlier post, I have tried to find out how much carbon dioxide is emitted in the process of manufacturing new cars. If this amount is substantial, it calls into question the GHG benefits of replacing inefficient cars with more efficient cars.
Honda reports that it emits less than a ton of carbon dioxide producing each vehicle. To put this number in perspective, a 20 MPG car will emit 50 tons of CO2 over 100,000 miles; a 30 MPG car will emit 33 tons of CO2. The ton of CO2 emitted when producing a new car is a trivial proportion of the CO2 emitted when driving.
Consistent with Honda's claim, this WRI report says that assembly accounts for 2% of total carbon emissions from cars.
The implication: The carbon emitted to produce a new car is not a compelling reason to oppose the Cash for Clunkers plan. Gas savings will make up for the manufacturing emissions in less than a year's worth of driving.
(May 7 Update: Bill Chameides, Dean of Duke's Nicholas School of the Environment, says that CO2 from manufacture is closer to 7 tons of CO2.)
See the blog archive on the right for newer posts on Cash for Clunkers.
Sunday, January 25, 2009
Cash for Clunkers is Being Weighed in Congress
[Update June 20 2009
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.
The final bill is linked to this post. The details below refer to an older bill.]
The Senate is now considering a Cash for Clunkers program, introduced by Dianne Feinstein, Susan Collins, and Charles Schumer. The bill has also been introduced in the House. The program works by subsidizing car owners who trade in a highly inefficient car for a car that is above average in efficiency.
Background
Jason Bordoff of the Brookings Institution recently made the case for a Cash for Clunkers program in a Detroit Free Press Column and a Brookings Institution Paper, as did Dean Baker of CEPR at Truthout. Alan Blinder proposed a Cash for Clunkers program in July 2008 and reviews similar programs that have been implemented in several states.
GPM makes clear why this is a brilliant idea--the gas savings and CO2 reduction of removing a 14 MPG for a 25 MPG car is huge (3 tons of carbon dioxide over 10,000 miles). There is no possible improvement that can be made to a 33 MPG to reduce carbon emissions by that amount. And, of course, it is a way of stimulating car purchases during the current economic crisis.
25 MPG doesn't seem like much. But removing the 14 MPG cars is extremely valuable, and as important today as winning the X prize with a 100 MPG car tomorrow.....
The math:
14 MPG = 700 gallons per 10,000 miles
25 MPG = 400 gallons per 10,000 miles
33 MPG = 300 gallons per 10,000 miles
The 14 MPG to 25 MPG improvement eliminates 300 gallons per 10,000 miles, or 3 tons of carbon dioxide.
The net carbon impact of Cash for Clunkers must also take into account CO2 emissions from car production. Honda emits less than a ton of carbon dioxide to produce each vehicle. Based on Honda's numbers, replacing a 14 MPG car with a new 25 MPG car would be carbon neutral after 4,000 miles of driving, and would represent a net carbon reduction after that. (Update: More on the carbon consequences of new cars at this later post.)
Eric De Place at Sightline Daily endorses the idea.
There are a few moral hazard issues with this proposal that I'll let the economists sort out (i.e., junkers sitting in a yard are fixed up to make a couple of thousand dollars, but the car is worth less than the bounty, and removing it yields no carbon reduction). The proposal addresses this issue by requiring that the car be registered in the previous 120 days.
If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.
The final bill is linked to this post. The details below refer to an older bill.]
The Senate is now considering a Cash for Clunkers program, introduced by Dianne Feinstein, Susan Collins, and Charles Schumer. The bill has also been introduced in the House. The program works by subsidizing car owners who trade in a highly inefficient car for a car that is above average in efficiency.
Background
Jason Bordoff of the Brookings Institution recently made the case for a Cash for Clunkers program in a Detroit Free Press Column and a Brookings Institution Paper, as did Dean Baker of CEPR at Truthout. Alan Blinder proposed a Cash for Clunkers program in July 2008 and reviews similar programs that have been implemented in several states.
GPM makes clear why this is a brilliant idea--the gas savings and CO2 reduction of removing a 14 MPG for a 25 MPG car is huge (3 tons of carbon dioxide over 10,000 miles). There is no possible improvement that can be made to a 33 MPG to reduce carbon emissions by that amount. And, of course, it is a way of stimulating car purchases during the current economic crisis.
25 MPG doesn't seem like much. But removing the 14 MPG cars is extremely valuable, and as important today as winning the X prize with a 100 MPG car tomorrow.....
The math:
14 MPG = 700 gallons per 10,000 miles
25 MPG = 400 gallons per 10,000 miles
33 MPG = 300 gallons per 10,000 miles
The 14 MPG to 25 MPG improvement eliminates 300 gallons per 10,000 miles, or 3 tons of carbon dioxide.
The net carbon impact of Cash for Clunkers must also take into account CO2 emissions from car production. Honda emits less than a ton of carbon dioxide to produce each vehicle. Based on Honda's numbers, replacing a 14 MPG car with a new 25 MPG car would be carbon neutral after 4,000 miles of driving, and would represent a net carbon reduction after that. (Update: More on the carbon consequences of new cars at this later post.)
Eric De Place at Sightline Daily endorses the idea.
There are a few moral hazard issues with this proposal that I'll let the economists sort out (i.e., junkers sitting in a yard are fixed up to make a couple of thousand dollars, but the car is worth less than the bounty, and removing it yields no carbon reduction). The proposal addresses this issue by requiring that the car be registered in the previous 120 days.
Sunday, January 11, 2009
GPM Calculator - With 2009 Car Information
MPG can be a misleading measure of fuel economy when comparing two cars. For a full explanation of why it is misleading, read the posts here and here. Focusing on gas consumption, and not MPG, gives you an accurate sense of fuel economy and carbon dioxide emissions.
If you are interested in a Cash for Clunkers trade in, use the calculator below or these charts to see the gas and cost savings of a trade.
The following calculator helps you convert MPG to "gallons per mile" (GPM) so that you can see the gas consumption of different cars. You can select a distance, gas price, and MPG level of your choice. You can compare the GPM of all new 2009 vehicles. Click the image to access the calculator.
If you are interested in a Cash for Clunkers trade in, use the calculator below or these charts to see the gas and cost savings of a trade.
The following calculator helps you convert MPG to "gallons per mile" (GPM) so that you can see the gas consumption of different cars. You can select a distance, gas price, and MPG level of your choice. You can compare the GPM of all new 2009 vehicles. Click the image to access the calculator.
The first calculator allows you to convert a car's MPG to GPM for an MPG level and distance of your choice. You will also be asked to enter a gas price to see the cost of driving that car for your chosen distance.
The second and third calculators allow you to compare the GPM of new 2009 vehicles. You will also be asked to enter a gas price to see the cost of driving the cars you are comparing. The MPG figures come from the EPA website fueleconomy.gov (which provides an option for comparing cars based on gallons per 100 miles).
If you are interested in downloading excel sheets and printable tables that convert MPG to GPM, go to the section on this webpage called Tools for calculating GPM from MPG.
Burning one gallon of gas releases about 20 pounds of carbon dioxide (another 20 percent is released in producing gasoline). Every 100 gallons saved reduces carbon dioxide emissions by 1 ton.
Update
Chris Shea summarized the argument for "Gallons per Mile" in the 2008 New York Times Magazine "Year in Ideas" issue.
Edmunds describes the shortcomings of MPG and offers a GPM calculator that lets you compare two specific car models in terms of "gallons per 100 miles" at Edmunds.com. (Note: Gallons per 100 miles is summarized as TFC for "True Fuel Consumption".)
If you would like a calculator that converts different combinations of metric (kilometers per liter, litres per 100 km), imperial gallons (as used in Britain), and US measures (such as MPG and gallons per 100 miles), this calculator and this calculator covers it all.
Monday, January 5, 2009
Teaching Materials
[Update September 2009: There are great "GPM" materials available here.]
The MPG Illusion can be a useful topic to discuss in a range of high school and college classes. It may be useful for:
Here are a few additional materials that overlap in part with the materials gathered at SPN:
Teaching notes - These notes focus on how to use quizzes and other materials to teach about the MPG Illusion in psychology classes. The MPG Illusion may also be of interest in economics and math classes.
Slides - These powerpoint slides contain quizzes, graphs and tables, results from the Science studies, and slides that connect the MPG Illusion to other topics.
Additional topics - These notes describe ways to use the MPG Illusion to lead into more general questions about greenhouse gas (GHG) decisions and about "nudges."
The following links are shorter, more memorable urls for the quiz, video, and calculator:
www.mpgquiz.com
www.mpgillusionvideo.com
www.gpmcalculator.com
Math teachers: The NSF has a special report on math education that includes the MPG Illusion.
The MPG Illusion can be a useful topic to discuss in a range of high school and college classes. It may be useful for:
- Showing the link between psychology and climate change in an intro psych class
- Showing framing effects (and debiasing) in a judgment and decision making class
- Discussing consumer decision making in a marketing or environmental economics class
- Illustrating a basic math concept (inverse relationships) with a consequential consumer decision.
Here are a few additional materials that overlap in part with the materials gathered at SPN:
Teaching notes - These notes focus on how to use quizzes and other materials to teach about the MPG Illusion in psychology classes. The MPG Illusion may also be of interest in economics and math classes.
Slides - These powerpoint slides contain quizzes, graphs and tables, results from the Science studies, and slides that connect the MPG Illusion to other topics.
Additional topics - These notes describe ways to use the MPG Illusion to lead into more general questions about greenhouse gas (GHG) decisions and about "nudges."
The following links are shorter, more memorable urls for the quiz, video, and calculator:
www.mpgquiz.com
www.mpgillusionvideo.com
www.gpmcalculator.com
Math teachers: The NSF has a special report on math education that includes the MPG Illusion.
Thursday, December 11, 2008
Wednesday, December 10, 2008
The case for GPM in a nutshell
Which is more useful to know: How far you can drive on a gallon of gas? Or, how much gas you will use while owning a car?
MPG answers the first question. It is useful when judging the range of one's gas tank. But it answers a less important question. GPM answers the question of gas consumption. We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions. MPG cannot.
People rely on subtraction when comparing MPG, which creates illusions. The improvements from 10 to 11 MPG, 16.5 to 20 MPG, and 33 to 50 MPG all save the same amount of gas over a given distance (e.g., 100 gallons per 10,000 miles). Given the inverse relationship between MPG and GPM, MPG requires division before subtraction (e.g., 1/20 - 1/16.5 or 100/20 - 100/16.5). GPM makes the magnitude of gas savings clear without additional math. GPM allows car buyers to use subtraction to compare the fuel economy of different cars (e.g., 500 vs. 400 gallons per 10,000 miles).
Providing a column of GPM numbers at Consumer Reports and at fueleconomy.gov would make accurate fuel economy comparisons far easier than the current column of MPG numbers. GPM needs to supplement MPG as a measure of fuel economy.
MPG answers the first question. It is useful when judging the range of one's gas tank. But it answers a less important question. GPM answers the question of gas consumption. We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions. MPG cannot.
People rely on subtraction when comparing MPG, which creates illusions. The improvements from 10 to 11 MPG, 16.5 to 20 MPG, and 33 to 50 MPG all save the same amount of gas over a given distance (e.g., 100 gallons per 10,000 miles). Given the inverse relationship between MPG and GPM, MPG requires division before subtraction (e.g., 1/20 - 1/16.5 or 100/20 - 100/16.5). GPM makes the magnitude of gas savings clear without additional math. GPM allows car buyers to use subtraction to compare the fuel economy of different cars (e.g., 500 vs. 400 gallons per 10,000 miles).
Providing a column of GPM numbers at Consumer Reports and at fueleconomy.gov would make accurate fuel economy comparisons far easier than the current column of MPG numbers. GPM needs to supplement MPG as a measure of fuel economy.
Monday, December 8, 2008
Summary of the MPG Illusion Studies
The MPG Illusion Studies (Originally posted at the Nudge Blog)
Follow this link to access the original Science article and online supplement.
Study 1: In the first study, college students were asked to rank each of the following vehicle changes (old car vs. new car) in terms of total gas saved, assuming that all the vehicles were driven 10,000 miles (shown in a random order):
A) 34 to 50 MPG
B) 18 to 28 MPG
C) 42 to 46 MPG
D) 16 to 20 MPG
E) 20 to 22 MPG
The majority of participants ranked the changes in order of the linear increase in improvement in mpg, (16 mpg for A, 10 mpg for B, etc.) However, in reality, B and D save more gas than A; D and E save more gas than C. Only 1 participant in 77 gave the correct order in terms of gas saved per 10,000 miles: B (198 gallons), D (125 gallons), A (94 gallons), E (38 gallons), C (30 gallons).
The A vs. B comparison is close to a family decision we made (to replace a minivan that got 18 mpg with a small station wagon, or to replace an efficient sedan with a hybrid compact). We were surprised to discover that option B saves twice as much gas as does A. Over 10,000 miles, B saves 198 gallons; A saves 94 gallons.
Study 2: A second study asked college students to price the gas savings from adding more efficient engines to a car that gets 15 mpg and costs $20,000, where the only feature that varies across vehicles was the mpg. Linear reasoning led them to undervalue improvements to 19 and 25 mpg and overvalue improvements to 55 mpg (under a range of discount rate assumptions).
Study 3: A third study showed that the mpg illusion could be broken by expressing efficiency as gallons per 100 miles (GPM). In this study, we asked a cross-section of adults to think about a town’s fleet of vehicles that all drove 10,000 miles per year. Half the vehicles in the fleet got 15 mpg and half got 34 mpg. Participants were asked to choose between 1) replacing the 15 MPG cars with vehicles that get 19 mpg, or 2) replacing the 34 MPG cars with vehicles that get 44 mpg.
Three-quarters preferred the second option when expressed as mpg. However, when gallons per 100 miles (GPM) information was also given, 64 percent correctly preferred the first option (replacing cars that got 6.67 gallons per 100 miles (GPM) with cars that got 5.26 GPM) to the second option (replacing cars that got 2.94 GPM with cars that got 2.27 GPM).
Option 1 (14 to 19 MPG) saves about 1.4 gallons per 100 miles compared to Option 2 (34 to 44 MPG), which saves only .7 for every vehicle replaced. In our scenario, Option 1 saves 14,035 gallons of gas per year; Option 2 saves only 6,684 gallons of gas per year.
Follow this link to access the original Science article and online supplement.
Study 1: In the first study, college students were asked to rank each of the following vehicle changes (old car vs. new car) in terms of total gas saved, assuming that all the vehicles were driven 10,000 miles (shown in a random order):
A) 34 to 50 MPG
B) 18 to 28 MPG
C) 42 to 46 MPG
D) 16 to 20 MPG
E) 20 to 22 MPG
The majority of participants ranked the changes in order of the linear increase in improvement in mpg, (16 mpg for A, 10 mpg for B, etc.) However, in reality, B and D save more gas than A; D and E save more gas than C. Only 1 participant in 77 gave the correct order in terms of gas saved per 10,000 miles: B (198 gallons), D (125 gallons), A (94 gallons), E (38 gallons), C (30 gallons).
The A vs. B comparison is close to a family decision we made (to replace a minivan that got 18 mpg with a small station wagon, or to replace an efficient sedan with a hybrid compact). We were surprised to discover that option B saves twice as much gas as does A. Over 10,000 miles, B saves 198 gallons; A saves 94 gallons.
Study 2: A second study asked college students to price the gas savings from adding more efficient engines to a car that gets 15 mpg and costs $20,000, where the only feature that varies across vehicles was the mpg. Linear reasoning led them to undervalue improvements to 19 and 25 mpg and overvalue improvements to 55 mpg (under a range of discount rate assumptions).
Study 3: A third study showed that the mpg illusion could be broken by expressing efficiency as gallons per 100 miles (GPM). In this study, we asked a cross-section of adults to think about a town’s fleet of vehicles that all drove 10,000 miles per year. Half the vehicles in the fleet got 15 mpg and half got 34 mpg. Participants were asked to choose between 1) replacing the 15 MPG cars with vehicles that get 19 mpg, or 2) replacing the 34 MPG cars with vehicles that get 44 mpg.
Three-quarters preferred the second option when expressed as mpg. However, when gallons per 100 miles (GPM) information was also given, 64 percent correctly preferred the first option (replacing cars that got 6.67 gallons per 100 miles (GPM) with cars that got 5.26 GPM) to the second option (replacing cars that got 2.94 GPM with cars that got 2.27 GPM).
Option 1 (14 to 19 MPG) saves about 1.4 gallons per 100 miles compared to Option 2 (34 to 44 MPG), which saves only .7 for every vehicle replaced. In our scenario, Option 1 saves 14,035 gallons of gas per year; Option 2 saves only 6,684 gallons of gas per year.
Tuesday, December 2, 2008
Graph of Gallons per 10,000 Miles (GPM) as a Function of MPG
Click here to open a powerpoint copy of this graph.
Burning one gallon of gas releases about 20 pounds of carbon dioxide (another 20 percent is released in producing gasoline). Every 100 gallons saved reduces carbon dioxide emissions by 1 ton.
Monday, December 1, 2008
The MPG Illusion in 2020
GPM shows that replacing the most inefficient cars (those with MPG in the teens) yields larger gas savings than improving already efficient cars. For example, convincing someone to trade in a 14 MPG car for a 20 MPG car reduces as much gas as having two people trade in a 33 MPG car for a 50 MPG car over the same distance.
Does the GPM argument apply only to today's highly inefficient cars? What if all cars in 2020 are "efficient" by 2009 standards (e.g., 50 MPG and above)? Is GPM still useful?
The answer is yes. Imagine that by 2020 cars range in MPG from 50 MPG (the Escalade superhybrid) to 170 MPG (the Prius superhybrid). GPM shows that the policy focus will always need to be on removing the most inefficient vehicles: Replacing a 50 MPG car with a 65 MPG car saves more gas (over a given distance) than replacing a 100 MPG car with a 170 MPG car.
Because of the curvilinear relationship between GPM and MPG, MPG will be potentially misleading even as cars become increasingly efficient. The benefits of thinking in terms of GPM will hold for all future efficiency levels, not just for today's SUVs.
Does the GPM argument apply only to today's highly inefficient cars? What if all cars in 2020 are "efficient" by 2009 standards (e.g., 50 MPG and above)? Is GPM still useful?
The answer is yes. Imagine that by 2020 cars range in MPG from 50 MPG (the Escalade superhybrid) to 170 MPG (the Prius superhybrid). GPM shows that the policy focus will always need to be on removing the most inefficient vehicles: Replacing a 50 MPG car with a 65 MPG car saves more gas (over a given distance) than replacing a 100 MPG car with a 170 MPG car.
Because of the curvilinear relationship between GPM and MPG, MPG will be potentially misleading even as cars become increasingly efficient. The benefits of thinking in terms of GPM will hold for all future efficiency levels, not just for today's SUVs.
Saturday, October 25, 2008
Tools for Calculating GPM from MPG
Update
For an online GPM calculator that includes conversions for all new 2009 cars, go to this post
This May 29 post has several tables that can help you calculate the gas savings from a Cash for Clunkers trade in.
Original Post
The following table shows how different levels of MPG translate to gallons of gas consumed over 10,000 miles (GPM). The MPG levels are chosen so that they translate to equal improvements in gas saving (100 gallons over 10,000 miles):
10.0 MPG = 1000 GPM (Gallons per 10,000 Miles)
11.0 MPG = 900 GPM
12.5 MPG = 800 GPM
14.0 MPG = 700 GPM
16.5 MPG = 600 GPM
20.0 MPG = 500 GPM
25.0 MPG = 400 GPM
33.0 MPG = 300 GPM
50.0 MPG = 200 GPM
The table makes clear that small MPG improvements on inefficient cars (e.g., 11 to 12.5, 14 to 16.5) save a large amount of gas. Replacing a 14 MPG car with a 25 MPG car saves more gas over a given distance than any possible improvement to a 33 MPG car. Greenhouse gas policy needs to focus on removing the most inefficient cars.
The following table shows how improvements of 5 MPG translate to gas consumption (gallons per 10,000 miles):
10 MPG = 1000 GPM (Gallons per 10,000 Miles)
15 MPG = 666 GPM
20 MPG = 500 GPM
25 MPG = 400 GPM
30 MPG = 333 GPM
35 MPG = 285 GPM
40 MPG = 250 GPM
45 MPG = 222 GPM
50 MPG = 200 GPM
This table makes clear the diminishing marginal returns to higher MPG.
These tables are summarized in this pdf file. Print it and keep it with you to calculate gas consumption when buying a new car.
An online GPM calculator can be found at this post. It also contains calculators for all new 2009 cars.
If you are interested in making GPM calculations for any distance of your choice, these excel files will do the math for you. They open in a new window and can be downloaded and saved to your computer:
GPM calculator for one car
GPM calculator for two cars
The second file (for two cars) is useful for (1) comparing the difference in efficiency between two cars, in which case one would use a single distance for both cars, and for (2) comparing total gas consumption for two cars in the same household, in which case one can let the distances of both cars vary to match expected driving.
If you find graphs helpful, the following picture shows the amount of gas used (on the y-axis) for different levels of MPG (shown as different lines) and for different distances of driving (on the x-axis). Click on the icon below to go to a two-page pdf file (click here for the pdf or here for a powerpoint file). The first page contains MPG values that range from 10 to 50; the second page contains MPG values for (relatively) efficient cars that range from 20 to 50.
For an online GPM calculator that includes conversions for all new 2009 cars, go to this post
This May 29 post has several tables that can help you calculate the gas savings from a Cash for Clunkers trade in.
Original Post
The following table shows how different levels of MPG translate to gallons of gas consumed over 10,000 miles (GPM). The MPG levels are chosen so that they translate to equal improvements in gas saving (100 gallons over 10,000 miles):
10.0 MPG = 1000 GPM (Gallons per 10,000 Miles)
11.0 MPG = 900 GPM
12.5 MPG = 800 GPM
14.0 MPG = 700 GPM
16.5 MPG = 600 GPM
20.0 MPG = 500 GPM
25.0 MPG = 400 GPM
33.0 MPG = 300 GPM
50.0 MPG = 200 GPM
The table makes clear that small MPG improvements on inefficient cars (e.g., 11 to 12.5, 14 to 16.5) save a large amount of gas. Replacing a 14 MPG car with a 25 MPG car saves more gas over a given distance than any possible improvement to a 33 MPG car. Greenhouse gas policy needs to focus on removing the most inefficient cars.
The following table shows how improvements of 5 MPG translate to gas consumption (gallons per 10,000 miles):
10 MPG = 1000 GPM (Gallons per 10,000 Miles)
15 MPG = 666 GPM
20 MPG = 500 GPM
25 MPG = 400 GPM
30 MPG = 333 GPM
35 MPG = 285 GPM
40 MPG = 250 GPM
45 MPG = 222 GPM
50 MPG = 200 GPM
This table makes clear the diminishing marginal returns to higher MPG.
These tables are summarized in this pdf file. Print it and keep it with you to calculate gas consumption when buying a new car.
An online GPM calculator can be found at this post. It also contains calculators for all new 2009 cars.
If you are interested in making GPM calculations for any distance of your choice, these excel files will do the math for you. They open in a new window and can be downloaded and saved to your computer:
GPM calculator for one car
GPM calculator for two cars
The second file (for two cars) is useful for (1) comparing the difference in efficiency between two cars, in which case one would use a single distance for both cars, and for (2) comparing total gas consumption for two cars in the same household, in which case one can let the distances of both cars vary to match expected driving.
If you find graphs helpful, the following picture shows the amount of gas used (on the y-axis) for different levels of MPG (shown as different lines) and for different distances of driving (on the x-axis). Click on the icon below to go to a two-page pdf file (click here for the pdf or here for a powerpoint file). The first page contains MPG values that range from 10 to 50; the second page contains MPG values for (relatively) efficient cars that range from 20 to 50.
Burning one gallon of gas releases about 20 pounds of carbon dioxide (another 20 percent is released in producing gasoline). Every 100 gallons saved reduces carbon dioxide emissions by 1 ton.
Other calculators can be found at these sites (we did not construct these calculators and have not used them extensively):
http://www.revolutioninmotion.com/fuel-cost-calculator.htm http://raja.gbc.googlepages.com/home (which is linked to the July 14 post at good republican usa)
Monday, October 13, 2008
"When the time is ripe for certain things, these things appear in different places" (Farkas Bolyai)
Updated May 2009
Others who noticed the problem with MPG or made a case for GPM before June 2008
Clark Williams Derry at Sightline Daily posted here, here, and here.
Eric De Place at Sightline Daily posted here and here, noted by Andrew Sullivan.
Barry Nalebuff and Ian Ayres in Forbes (Why Not?), Dennis Simanaitis at Road and Track here (see halfway down) and here, Automotive News, Boston Herald, Ben Garrido at Reno News & Review, Tony's Climate Blog, Halfbakery, No, Dave, it's just you,
A CarTalk puzzler that illustrates the problem with MPG
Discussion of the MPG Illusion or GPM between June and November 2008
See this collection of links on my Research Highlights webpage.
Discussion of the MPG Illusion or GPM since December 2008
New York Times, Autobloggreen (and here, here, here, and here), New York Times - Greenwire, Green Car Reports here and here, Christian Science Monitor, Good Magazine, Detroit Free Press, Harvard Gazette, Felix Salmon-Reuters, Treehugger, Consumerology, W&M Alumni Magazine, Climate Progress, The Street (AP), Next 100, IEEE Spectrum, Marty Padgett at the Car Connection, Huffington Post, Cognitive Daily, Wattzon, After Gutenberg, BMSeer, Driver Side, Fat Knowledge, National Motorists Association, Hybrid Review here and here, Whipnotic, Science in Society, Metamodern, Master Resource, Lemon Laws, Earth2Tech, Overcoming Bias, EnerBlog, Distributor Cap NY, Will Wilson, Business is Personal, My Black Brick, A Blue View, McKenna VW, Enviroboys, Loyalty Driver,
Others who noticed the problem with MPG or made a case for GPM before June 2008
Clark Williams Derry at Sightline Daily posted here, here, and here.
Eric De Place at Sightline Daily posted here and here, noted by Andrew Sullivan.
Barry Nalebuff and Ian Ayres in Forbes (Why Not?), Dennis Simanaitis at Road and Track here (see halfway down) and here, Automotive News, Boston Herald, Ben Garrido at Reno News & Review, Tony's Climate Blog, Halfbakery, No, Dave, it's just you,
A CarTalk puzzler that illustrates the problem with MPG
Discussion of the MPG Illusion or GPM between June and November 2008
See this collection of links on my Research Highlights webpage.
Discussion of the MPG Illusion or GPM since December 2008
New York Times, Autobloggreen (and here, here, here, and here), New York Times - Greenwire, Green Car Reports here and here, Christian Science Monitor, Good Magazine, Detroit Free Press, Harvard Gazette, Felix Salmon-Reuters, Treehugger, Consumerology, W&M Alumni Magazine, Climate Progress, The Street (AP), Next 100, IEEE Spectrum, Marty Padgett at the Car Connection, Huffington Post, Cognitive Daily, Wattzon, After Gutenberg, BMSeer, Driver Side, Fat Knowledge, National Motorists Association, Hybrid Review here and here, Whipnotic, Science in Society, Metamodern, Master Resource, Lemon Laws, Earth2Tech, Overcoming Bias, EnerBlog, Distributor Cap NY, Will Wilson, Business is Personal, My Black Brick, A Blue View, McKenna VW, Enviroboys, Loyalty Driver,
Wednesday, October 8, 2008
The MPG Illusion and the need for GPM
The MPG Illusion
Test your understanding of fuel efficiency with this interactive quiz.
Go here for the original Science article.
This March 2009 post offers a current overview.
The Problem with MPG
What is the problem with MPG? Consider a decision between two cars--a current vehicle and a new vehicle that is more efficient. Which improvement will save the most gas over 10,000 miles?
A) An improvement from 10 to 11 MPG
B) An improvement from 16.5 to 20 MPG
C) An improvement from 33 to 50 MPG
Surprisingly, all save about the same amount of gas over 10,000 miles: About 100 gallons.
The way to calculate the amount of gas used is to divide distance by MPG. A quick check of the numbers above will confirm the following gas usage over 10,000 miles:
10 MPG = 1000 gallons
11 MPG = 900 gallons
16.5 MPG = 600 gallons
20 MPG = 500 gallons
33 MPG = 300 gallons
50 MPG = 200 gallons
We want to emphasize that a higher MPG car is always more efficient than a lower MPG car for a given distance. We are not saying that a car that getst 11 MPG is somehow better than a car that gets 50 MPG -- to the contrary, we encourage all drivers to buy the most efficient vehicle they can. What we are saying is that MPG can be confusing when thinking about the benefits of improving MPG. The bottom line is that equal increases in MPG are not equal in gas savings.
As the examples above shows, small MPG improvements on inefficient cars can save a lot of gas. Of course, most people look at an improvement from 10 to 11, or 16 to 20, and think, why bother? But replacing an inefficient car with a car that is more efficient -- even by just a few miles per gallon -- is valuable in both gas savings and greenhouse gas reductions. Every 100 gallons saved reduces carbon dioxide emissions by 1 ton.
In short, you cannot simply look at an MPG increase from one vehicle to another to know the gas savings. Also, when a family thinks about its average fuel consumption, it cannot simply take an average MPG levels of two vehicles. Given two cars that are driven the same distance, the combination of 18 MPG and 50 MPG uses more gas than the combination of 28 MPG and 30 MPG. Direct comparisons of MPG is what leads to illusions. In each case, you have to convert MPG to know the amount of gas used.* We describe this step next.
The Solution
The solution to this illusion is thinking about gallons of gas used over some meaningful distance. We will use the term GPM (gallons per mile) as a general shorthand for expressing gas consumption over a given distance. We will focus specifically on Gallons Per 10,000 Miles.
We favor 10,000 miles for several reasons. First, 10,000 miles is close to the distance many people drive in a year. Second, it is a round number that is easy to adjust up or down. Third, it overcomes a natural tendency to minimize small gains: What appear to be small gas savings at 100 miles (6 vs. 5 gallons per 100 miles) are more obviously worthwhile when aggregated to a yearly number (600 vs. 500 gallons per 10,000 miles). The value of saving 100 gallons per year is clear. (The effect of scaling on "discriminability" is discussed in a paper that is available by request.)
Of course, because people do drive different distances in a year, final GPM numbers need to be tailored for each person's own circumstance. The tables and calculators below do the GPM math for you.
The Key is Amount of Gas Used
Another way of framing the basic issue is "Which is more useful to know: How far you can drive on a gallon of gas? Or, how much gas you will use while owning a car?" MPG answers the first question. GPM answers the second question.
We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions. MPG cannot.
Test your understanding of fuel efficiency with this interactive quiz.
Go here for the original Science article.
This March 2009 post offers a current overview.
The Problem with MPG
What is the problem with MPG? Consider a decision between two cars--a current vehicle and a new vehicle that is more efficient. Which improvement will save the most gas over 10,000 miles?
A) An improvement from 10 to 11 MPG
B) An improvement from 16.5 to 20 MPG
C) An improvement from 33 to 50 MPG
Surprisingly, all save about the same amount of gas over 10,000 miles: About 100 gallons.
The way to calculate the amount of gas used is to divide distance by MPG. A quick check of the numbers above will confirm the following gas usage over 10,000 miles:
10 MPG = 1000 gallons
11 MPG = 900 gallons
16.5 MPG = 600 gallons
20 MPG = 500 gallons
33 MPG = 300 gallons
50 MPG = 200 gallons
We want to emphasize that a higher MPG car is always more efficient than a lower MPG car for a given distance. We are not saying that a car that getst 11 MPG is somehow better than a car that gets 50 MPG -- to the contrary, we encourage all drivers to buy the most efficient vehicle they can. What we are saying is that MPG can be confusing when thinking about the benefits of improving MPG. The bottom line is that equal increases in MPG are not equal in gas savings.
As the examples above shows, small MPG improvements on inefficient cars can save a lot of gas. Of course, most people look at an improvement from 10 to 11, or 16 to 20, and think, why bother? But replacing an inefficient car with a car that is more efficient -- even by just a few miles per gallon -- is valuable in both gas savings and greenhouse gas reductions. Every 100 gallons saved reduces carbon dioxide emissions by 1 ton.
In short, you cannot simply look at an MPG increase from one vehicle to another to know the gas savings. Also, when a family thinks about its average fuel consumption, it cannot simply take an average MPG levels of two vehicles. Given two cars that are driven the same distance, the combination of 18 MPG and 50 MPG uses more gas than the combination of 28 MPG and 30 MPG. Direct comparisons of MPG is what leads to illusions. In each case, you have to convert MPG to know the amount of gas used.* We describe this step next.
The Solution
The solution to this illusion is thinking about gallons of gas used over some meaningful distance. We will use the term GPM (gallons per mile) as a general shorthand for expressing gas consumption over a given distance. We will focus specifically on Gallons Per 10,000 Miles.
We favor 10,000 miles for several reasons. First, 10,000 miles is close to the distance many people drive in a year. Second, it is a round number that is easy to adjust up or down. Third, it overcomes a natural tendency to minimize small gains: What appear to be small gas savings at 100 miles (6 vs. 5 gallons per 100 miles) are more obviously worthwhile when aggregated to a yearly number (600 vs. 500 gallons per 10,000 miles). The value of saving 100 gallons per year is clear. (The effect of scaling on "discriminability" is discussed in a paper that is available by request.)
Of course, because people do drive different distances in a year, final GPM numbers need to be tailored for each person's own circumstance. The tables and calculators below do the GPM math for you.
The Key is Amount of Gas Used
Another way of framing the basic issue is "Which is more useful to know: How far you can drive on a gallon of gas? Or, how much gas you will use while owning a car?" MPG answers the first question. GPM answers the second question.
We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions. MPG cannot.
Tools for GPM Calculations
Follow these links for tools for calculating GPM:
Tools for Calculating GPM from MPG
GPM Calculator Including All 2009 AutomobilesMPG vs. GPM - Which is more Useful?
Does using GPM imply that MPG should be scrapped? No. MPG is useful. Specifically, MPG tells you the range of your car's gas tank. For example, MPG can help you decide whether you can wait two more exits to refill your tank.
Both MPG and GPM have a useful role at different points in owning a car. MPG is useful when you're driving a car. GPM is useful when you're purchasing a car -- it better captures the fuel consumption, and fuel savings, when comparing a current car to a new car, or when comparing two new cars to each other.
Note that both measures serve equally well to tell you what is more efficient: 50 MPG is better than 20 MPG; 200 gallons per 10,000 miles is better than 500 gallons per 10,000 miles. They are not equal, however, in accurately conveying the gas savings from efficiency gains.
The two measures do not provide equal information. We would argue that GPM is better than MPG at helping people see the outcomes of their car decisions:
- GPM spells out in clear numbers how much gas one is going to use. 1,000 gallons per 10,000 miles is clearly dreadful. 200 gallons per 10,000 is clearly great.
- One can immediately tell how much a car will cost to fuel over 10,000 miles.
- One can see the actual magnitude of the gas savings when comparing a more efficient car to a less efficient car. Specifically, one can subtract one car's GPM from another to see the gas savings. MPG cannot be subtracted.
None of these outcomes is apparent with MPG until you do more math.
The Research Findings
Our main research finding is that the majority of people assume that equal increases in MPG are equal in gas savings; a minority thinks that gas savings are equivalent to percentage improvement. Both lines of reasoning lead to erroneous conclusions. In a final study we show that expressing fuel efficiency as GPM (in this case, gallons per 100 miles) leads the majority of people to identify the efficiency improvements that save the most gas. A brief summary of the three studies in Science appears at the Nudge blog. (and reposted here)
The Cause of the Illusion
For those mathematically inclined, the cause of the illusion is simple: MPG creates an illusion because it is a ratio. By necessity, MPG has a curvilinear relationship with its inverse (GPM). Because people do not spontaneously take the reciprocal, they incorrectly map changes in MPG to changes in amount of gas consumed. The formula for calculating GPM in this graph is 10,000 miles divided by MPG. Download a powerpoint copy of this graph here.
Percentage Improvement in MPG is Flawed
Some people expect that, although linear reasoning with MPG is incorrect, percentage increases in MPG captures amount of gas saved over a given distance. Even percentages, however, are prone to illusions with MPG. See this note for three examples of why percentage improvement fails.
For example, it is easy to see in the example given above that improving from 10 to 11 MPG is a 10% improvement; 16.5 to 20 MPG is a 20% improvement; and 33 to 50 MPG is a 50% improvement. Although they all represent different percentage improvements in MPG, they all save 100 gallons of gas over 10,000 miles. Although an improvement from 10 MPG to 13 MPG is only a 30% improvement, it saves more than twice the gas of the 50% improvement from 33 to 50 MPG. The problem with percentage reasoning is that it needs to be applied to a starting level of gas consumed; that amount differs over different levels of MPG. It is captured in GPM.
Imperial vs. Metric
The metric system does not solve the MPG illusion. India uses kilometers and liters but expresses efficiency as kilometers per liter. Because the ratio is distance over volume, it creates a parallel illusion to MPG. See this blog for a nice translation to the Indian context:
http://www.livemint.com/2008/06/19222458/Efficiency-measure-gives-wrong.html
Many countries currently use liters per 100 kilometers, which has the right numerator (volume) and denominator (distance) for judging efficiency gains. However, some people living in those countries have questioned how helpful it has been. We think that the base distance should be larger so that differences between efficiency levels are clearer and involve fewer decimal places.
Go here for more thoughts on the metric system.
*Technically speaking, GPM is an intermediate step in calculating the harmonic mean used to measure automaker compliance with CAFE standards. A family also needs to calculate a harmonic mean to understand their total fuel efficiency-the family can't simply weight the MPG of two vehicles by their respective driving distances.
My main webpage at Duke can be found here
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