Wednesday, September 30, 2009

DOE Report on Improving Fuel Economy Decisions

The Department of Energy has a comprehensive August 2009 draft report available for the public entitled:

Reducing Oil Use and CO2 Emissions by Informing Consumers’ Fuel Economy Decisions: The Role for Clean Cities
A Discussion Paper for Clean Cities Coalitions and Stakeholders to Develop Strategies for the Future

The authors describe the fact that consumers are confused about the relationship between fuel economy (miles per gallon) and fuel consumption (gallons per mile), which leads to inaccurate assessments of the value of fuel economy.

Later in the report (pages 15-16), they note that

Another apparent source of confusion is a consequence of reporting fuel economy in miles per gallon rather than fuel consumption in gallons per 100 miles. Research has shown that many consumers perceive the value of an increase in 10 MPG from 40 to 50 MPG to be equal to a 10 MPG increase from 20 to 30 MPG. In fact, the increase from 40 to 50 MPG reduces fuel consumption by 0.50 gallons per 100 miles while an increase from 20 to 30 reduces fuel consumption by 1.67 gallons per 100 miles. For a vehicle traveling 10,000 miles and gasoline at $2.50 per gallon, the difference in annual fuel savings is almost $300. The website provides the option to view fuel economy estimates in terms of miles per gallon, gallons per 100 miles or liters per 100 kilometers. However, this facility is little used by visitors to the website. Additional market research may lead to better strategies for helping consumers understand this important distinction.
We agree that consumers will generally not select "gallons per 100 miles" on their own--although some may have heard about the problems with MPG comparisons, the vast majority have not.

That's why in an earlier post we recommended the following for the website and the Fuel Economy Guide run by the EPA, DOE, and EERE:

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 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
Both USA Today and Popular Mechanics have started using the "gallons per 100 miles" measure.

Currently offers a "gallons per 25 miles" measure automatically (see below).

The logic for choosing this distance is that it is a typical daily drive. Jack Soll and Dan Feiler have pointed out that this number may be useful for daily driving decisions (whether to make a trip or not). The problem with this measure, however, is at the time of the car purchase decision. It makes all gas consumption numbers look small and similar (.5 vs .64 vs 1.04). At the time of a car purchase, a longer distance is more realistic and makes differences between cars clearer.

In other research, my coauthors and I have shown that expanded scales lead consumers to use that dimension more in their decision. The more expanded the scale the more weight people give it in their decision making. In this case, gallons per 25 miles is a contracted scale and gallons per 100 miles (or gallons per 10,000 miles) is expanded. For example, the numbers in the screenshot above translate to:


We think that car buyers would be served best by gallons per 10,000 miles. This could be listed at, on car stickers, in Consumer Reports, etc. Offering gallons per 10,000 miles during the car purchase decision would:

1) give buyers a more accurate understanding of fuel savings (compared to MPG) and

2) make buyers more sensitive to the benefits of fuel savings (compared to gallons per 25 miles).

Sunday, September 27, 2009

Ask Marilynn

"Jack drives a gas-guzzling SUV 15,000 miles a year. It averages 10 mpg. His wife Jill drives an economy car the same distance. It gets 30 mpg. The couple wants to use less gasoline. They can afford to trade Jack’s clunker for an SUV that gets 13 mpg or trade Jill’s sensible car for a hybrid that gets 60 mpg.

Which trade would save the most gasoline?
—Michael McClendon, Austin, Tex.

Test your intuition before doing the math. Look at the choices and make an educated guess about what makes the most sense. The answer appears at the end of the column"

Saturday, September 26, 2009

When Consumer Reports Does Math...

The author of Hybrid Car Review spotted a great example of poor MPG math from the people who have the knowledge and power to do better. (See more analysis at ecommoder or the comments at autobloggreen, too.) Here is the post:

"It's stories and quotes like this that prove the point that some have been making about switching from mpg to gallons per mile (gpm).

Consumer Reports

The Honda Insight hybrid showed the largest drop in fuel economy—over 15 mpg going from 55 to 75 mph
Vehicles with lower fuel efficiency had the smallest drop. The V8-powered Mercury Mountaineer has a fuel economy of 23.8 mpg at 55 mph and that drops to 21.2 mpg at 65 and 17.8 at 75 mph..
That is just so misleading. In gallons per mile, the Insight drops from 1.93 gallons per 100 miles to 2.74 gpm. That's a difference of 0.81 gpm. The Mountaineer drops from 4.20 gpm to 5.62 gpm, a difference of 1.42 gpm.

So not only are starting off at twice the inefficiency, you end up losing almost twice as much.

This is exactly the misleading nonsense a switch from mpg to gpm would fix."


PS, recommended math materials

Gallons Per Mile Teaching Materials

Claudia Bode at the University of Kansas and colleagues have put together an extensive, well-designed set of teaching materials to help students see the problems with "miles per gallon" and gain insights from "gallons per mile."

These materials are free to the public. They include both online pages and downloadable pdfs. The materials cover a wide range of topics on fuel efficiency, including the issue of driving speed.

Here are the two links to

Here is a link to teaching materials we prepared in early 2009.