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

By Julie Steenhuysen

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.