Saturday, March 14, 2009

Overview of GPM - Updated

Know Your GPM

Know your gas consumption and compare it to any 2009 car here.


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:

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 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.