U.S. drivers should think in gallons per mile: report

CHICAGO (Reuters) - If soaring gasoline prices have prompted you to look for a more fuel-efficient ride, using miles per gallon as a guide could lead you astray, U.S. researchers said on Thursday.

Instead, they propose a new standard based on gallons per mile, which gives people a far better idea of how much gasoline they might save by switching trading in that gas-guzzling minivan.

"There is a math illusion here," said Richard Larrick, a management professor at Duke University, whose research appears in the journal Science.

Larrick said most people think improvements in miles per gallon are all the same, where a 5 gallon per mile improvement would yield the same gas savings in a car that gets 10 miles per gallon or 20 miles per gallon. (One mile equals 1.61 kilometers, and one U.S. gallon equals 3.79 liters.)

"The reality that few people appreciate is that improving fuel efficiency from 10 to 20 miles per gallon is actually a more significant savings than improving from 25 to 50 miles per gallon for the same distance of driving," Larrick said.

He tested this out in a number of different experiments on U.S. college students.

When presented with a series of car choices in which fuel efficiency was defined in miles per gallon, the students could not easily identify the choice that would result in the greatest gains in fuel efficiency, he said.

People had a much easier time when fuel efficiency was expressed in gallons per 100 miles. In that case, a car that gets 18 miles per gallon uses 5.5 gallons of gas per 100 miles, and a car that gets 28 miles per gallon uses just 3.6 gallons per 100 miles. With gasoline prices over $4 a gallon, that's a difference of about $8 per 100 miles.

"If we just turn everything around, you can see where are the large savings in gallons of gas," Larrick said in a telephone interview. The idea is not new. Many other countries, especially in Europe, already use a standard that compares gas used per trip.

To translate miles per gallon into gallons per 10,000, Larrick said people can simply divide 10,000 by miles per gallon. Cars with the highest miles per gallon are always the most fuel efficient, he said. It is when people are trying to replace a car that they may be misled.

That's how he became interested in this problem.

"We were trying to decide whether to get rid of a minivan and go for a station wagon versus getting rid of a sedan and going for a really high-mileage hybrid car," Larrick said.

"We realized in the end we were better off trading in the minivan and only gaining 10 miles per gallon then we would be trying to swap out the sedan for a highly efficient car."

To help make these choices easier, Larrick and colleagues recommend consumer publications and car makers start listing fuel efficiency in terms of gallons per 10,000 miles driven, which he said is roughly the distance people in the United States drive in a year.

Larrick's team has developed a conversion table that can be found here

By Julie Steenhuysen
(Editing by Will Dunham)