Tuesday, February 23, 2010

Airplane MPG

In November, there was a nice post on R-Squared Energy Blog about airplane fuel efficiency.

Robert Rapier writes: "Often when I am flying, I think about the amount of fuel that the airplane is burning. Then when I am off the plane, I usually forget about it. I have heard mixed opinions on the overall efficiency of airline travel versus automobile travel, but just never got around to investigating the matter myself....."

After reviewing a bit of math he concludes that his per capita fuel economy on a recent 757 flight was 81 miles per gallon.

And then he notes, "Of course it is important to note that while the fuel economy looks pretty good, the miles traveled are very high relative to automotive transportation. I generally travel less than 5,000 miles per year with my car, so if I drive a car that gets 25 miles per gallon it would only take about 16,000 miles on an airplane to equate to an entire year's consumption in my car."

When comparing airplane and car fuel efficiency, there are a couple of allures of flying:

First, the MPG of flying is better than any car. But as Robert Rapier notes, many people fly much further than they drive. One could easily sweat the details of a car purchase--trading in a 30 MPG car for a Prius--to be green, but emit the equivalent annual CO2 savings on two round trip flights from the east coast to the west coast.

I created these graphs a couple of years ago to get a better appreciation of how distance and MPG tradeoff.
(The maximum MPG in these graphs is 50.)

The second allure of flying is that the plane was going to fl;y anyway so my marginal CO2 contribution is zero. But this calculation ignores the cumulative effect of individual demand. Higher demand in period t leads to more scheduled flights in t+1. It is an interesting social dilemma where each individual can feel like he or she is flying for free, but the plane wouldn't be flying if there were no passengers. Somewhere between the average CO2 contribution and the marginal CO2 contribution is the "true value".

Thursday, February 18, 2010

MPG, VW, and Honey Bees

An NPR story this week celebrated the incredible "fuel efficiency" of honey bees. They compared the energy efficiency of bee flight to a new VW vehicle called the L1.

The VW will purportedly get 170 miles per gallon.

The bee? 4,704,280 MILES PER GALLON!

What if we could magically convert the new VW to bee-level efficiency? How valuable would that be in gas savings? Let's look at gas consumption measured as gallons per 10,000 miles:

The VW L1= 58.823 gallons per 10,000 miles

The bee = .002 gallons per 10,000 miles
Replacing the VW with a bee would save 58 gallons over 10,000 miles.

Let's put this amount of gas savings in perspective. This same gas savings is achieved by the following trade ins (also over 10,000 miles of driving):

A 12 MPG vehicle for a 13 MPG vehicle
A 20 MPG vehicle for a 23 MPG vehicle
A 30 MPG vehicle for a 36 MPG vehicle

Here's another way to look at it:

The VW is about 7 times more efficient than an average car today that gets about 25 mpg. The bee is 200,000 times more efficient than today's average car! The efficiency of the bee seems to dwarf the efficiency of the VW.

And here's another way to look at it:

The ratio of the VW to the bee is miniscule. 170 MPG is less than .01% of 5 million MPG. It seems like nothing.

But let's put the 170 MPG in perspective. MPG misleads in this regard because it is gas consumption, not mileage, that matters.

If you drive today's average car 10,000 mile, you will use about 400 gallons of gas.

The VW will use 59 gallons of gas over the same distance. This saves roughly 340 gallons compared to today's car.

And the bee will use just 2/thousandths of a gallon and saves close to 400 gallons compared to today's car.

Surprisingly, although the VW MPG is less than .01% of the bee MPG, the VW MPG achieves 85% of the gas savings from Bee-level efficiency.

This is a lesson in why national policy needs to focus on gas consumption, not mileage: The important gas savings come from improving the efficiency of low MPG vehicles (e.g., trade ins of 14 MPG cars for 25 or 30 MPG cars). Replacing a 14 MPG car with a 27 MPG car saves more gas than replacing a 33 MPG car with a bee! Large MPG improvements on efficient cars do little to reduce gas consumption. Measures such as "gallons per 10,000 miles" makes this distinction clear.

Let's beware the obsession with large MPG numbers. Let's focus on improving low MPG vehicles.

Monday, February 8, 2010

Why GPM -- A Brief Review

Autobloggreen has a post on the New York State Senate bill that would require car dealers to describe fuel efficiency as "gallons per 1,000 miles". The post has prompted the familiar call for using the metric system (see this post on the connection between "GPM" and the metric system) and the familiar lament that people should understand the math.

A comment by "Throwback" on Autobloggreen reads:

Throwback 2:40PM (2/08/2010)

Another wasteful bill. What is the purpose? You don't think people understand that if they buy a car that gets 25 mpg vs 20 mpg they will be using less fuel? As a native New Yorker I am embarrassed by the (lack of) quality of the states politicians.

Here's a short answer to Throwback:

Yes, higher MPG is better than lower MPG, and people recognize this.

But people use the difference in MPG as the rough approximation of gas savings, which is misleading--often in a big way. Instead of subtracting MPG, car buyers need to first divide a given distance by each car's MPG, and then subtract. (See the last half of this post on the math.) That's what GPM does without effort--a standardized GPM measure can be subtracted to know actual gas savings.

Consider two trade ins:

A) 30 MPG to 45 MPG
B) 15 MPG to 20 MPG

Impressed by the 15 MPG improvement in option A? (A 50% improvement in MPG.)

Unimpressed by the 5 MPG improvement in option B? (A 33% improvement in MPG.)

Option A saves 11 gallons per 1000 miles; Option B saves 17 gallons per 1000 miles.

The New York bill is designed to highlight the gas savings available to people who are driving or considering cars in the teens. This is where the biggest savings are possible.

In fact, GPM may be most effective at keeping people who are currently driving more efficient cars from opting for less efficient ones. MPG tempts us to think that there is little harm in trading in a 20 MPG minivan for a 15 MPG SUV. What's 5 MPG? GPM makes clear the impact.

Compare MPG to "Gallons per 1000 miles" below:

MPG Gallons per 1000 miles
10 100
11 91
12 83
13 77
14 71
15 67
16 63
17 59
18 56
19 53
20 50
21 48
22 45
23 43
24 42
25 40
26 38
27 37
28 36
29 34
30 33
31 32
32 31
33 30
34 29
35 29
36 28
37 27
38 26
39 26
40 25
41 24
42 24
43 23
44 23
45 22
46 22
47 21
48 21
49 20
50 20

Friday, February 5, 2010

Gallons per Mile Bill Clears New York Senate Committee

[Update April 21: The GPM bill passed the New York Senate yesterday as part of larger Earth Day legislative package. See this post for details. The original bill, described below, required that dealers provide "gallons per 1,000 miles" for each vehicle they sold. The new bill requires that dealership display a chart translating mpg to "gallon per 1,000 miles".]

The New York Senate Environmental Conservation Committee has passed a new fuel efficiency bill that includes a "gallons per mile" requirement. The bill requires that vehicle manufacturers list "gallons per 1,000 miles" for city, highway, and combined driving.

The bill was championed by Senator Daniel Squadron, who laid out his rationale in this December article, and received broad support from environmental groups:
"Urging the passage of Senator Squadron’s bill were the New York League of Conservation Voters, the Environmental Defense Fund, the Natural Resources Defense Council, the Sierra Club’s Atlantic Chapter, and Senator Antoine Thompson, the Chair of the Environmental Conservation Committee."
The blog All over Albany lays out the case in more detail.

We believe this low cost change for presenting fuel efficiency information is of great value to consumers and the environment. We applaud Senator Squadron for endorsing it.

Although we advocated for "gallons per 100 miles" and "gallons per 10,000 miles" as useful standards in the supplement to the Science article, we note the benefits of "gallons per 1,000 miles":
  1. 1,000 miles is roughly what the average American drives in a month, so it is a meaningful number
  2. It allows easy estimation of yearly consumption (multiply by 10, roughly)
  3. It avoids the problem of seemingly small differences in efficiency that occurs when comparing "gallons per 100 miles"
Barry Nalebuff and Ian Ayres made the case for gallon per 1,000 miles in this Why Not? column for Forbes magazine.

Here is an excerpt from the bill (the full bill can be found here):


An act to amend the environmental conservation law, in relation to requiring automobile dealers display a fuel economy label on all new vehicles

To require that new passenger motor vehicles sold in New York State post a "gallons-per-mile fuel impact statement."

Adds a new section 19-1104 to the environmental conservation law to require vehicle manufacturers to display a gallons per thousand miles fuel impact statement. Such statement shall set forth the average number of gallons the vehicle is expected to use when traveling a distance of one thousand miles of city mileage, highway mileage and combined city and highway mileage. The bill provides for a civil fine of not more than $100 per vehicle to be imposed on manufacturers for a violation of this section.

Gallons per miles driven is a much more useful means of measuring fuel efficiency than the current miles per gallon standard. It enables a vehicle purchaser to more easily compare the fuel efficiency between various models of automobiles. This bill, which would require manufacturers to display the average gallons per one thousand city, highway and combined miles, will allow consumers to know accurately at a glance the cost of operating a vehicle over one thousand miles. Additionally, this allows environmentally conscious consumers to identify the relative environmental effect of different vehicles. Encouraging consumers to buy more fuel efficient vehicles can help save money, reduce greenhouse gas emissions, improve energy security and oil dependence costs, and increase energy sustainability.

The MPG Illusion among Transportation Professionals

Are transportation experts immune to the MPG Illusion?

In a presentation at the 2010 Transportation Research Board Conference entitled Mile-per-Gallon Illusions and CAFE Distortions: When Even Transport Experts Have Trouble, Dana Rowan, Alex Karner, and Debbie Niemeier of UC Davis report that transportation professionals make better judgments of fuel efficiency gains using "gallons per 100 miles" than using MPG.