Quiz time: Which of the following would save more fuel?

a) Replacing a compact car that gets 34 miles a gallon with a hybrid that gets 54 m.p.g.

b) Replacing from an S.U.V. that gets 18 m.p.g. with a sedan that gets 28 m.p.g.

c) Both changes save the same amount of fuel.

The correct answer is b. In fact, going from 18 m.p.g. to 28 m.p.g. saves more than twice as much fuel as going from 34 m.p.g. to 54 m.p.g. (198 gallons vs. 94 gallons), according to two management professors from Duke University.

Richard Larrick and Jack Soll ran a series of experiments to show that the current standard of miles per gallon leads consumers to believe that fuel consumption is reduced at an even rate as efficiency improves. But that’s not the case.

“Miles per gallon is misleading and can play tricks on our intuitions,” Prof. Soll said in a press release, which also links to an interactive quiz.

Profs. Soll and Larrick offered an alternative metric: gallons per mile. Expressed in gallons used per 100 miles, 18 m.p.g. becomes 5.5 gallons per 100 miles, and 28 m.p.g. becomes 3.6 gallons per 100 miles. And the difference is suddenly obvious: nearly two gallons every 100 miles, or a difference of more than $8.

Many European countries already list efficiency in terms of volume per distance traveled (liters per 100 kilometers or L/100km). The Duke professors recommend that American automakers and consumer publications list efficiency in terms of gallons per 10,000 miles. Thus, 10 m.p.g. becomes 1,000 gallons per 10,000 miles, and 25 m.p.g. becomes 400 gallons per 10,000 miles.

Profs. Soll and Larrick offer a better illustration: If you switched from a 12 m.p.g. S.U.V. to a 15 m.p.g. S.U.V., you would save approximately 167 gallons of fuel over 10,000 miles. At a cost of $4 a gallon, that’s roughly $700.

“This measure makes it easy to see how much gas one might use in a given year of driving and how much gas, and money, can be saved by opting for a car with greater efficiency,” Prof. Larrick said.