As Americans focus on how to keep gas in their cars, a new study is giving them even more to think about.

Forget “miles per gallon”, and think instead “gallons per mile,” say management professors Richard Larrick and Jack Soll from Duke University’s Fuqua School of Business.

The professors say posting a vehicle’s fuel efficiency in those terms would help consumers make better decisions about car purchases and environmental impact.

Larrick and Soll were inspired by debates they had while carpooling in a hybrid car. They ran a series of experiments showing that the current standard, miles per gallon or mpg, leads consumers to believe that fuel consumption is reduced at an even rate as efficiency improves. People presented with a series of car choices in which fuel efficiency was defined in miles per gallon were not able to easily identify the choice that would result in the greatest gains in fuel efficiency.

One example is that most people ranked an improvement from 34 to 50 mpg as saving more gas over 10,000 miles than an improvement from 18 to 28 mpg, even though the latter saves twice as much gas. (Going from 34 to 50 mpg saves 94 gallons; but from 18 to 28 mpg saves 198 gallons).

These mistaken impressions were corrected, however, when participants were presented with fuel efficiency expressed in gallons used per 100 miles rather than mpg. Viewed this way, 18 mpg becomes 5.5 gallons per 100 miles, and 28 mpg is 3.6 gallons per 100 miles — an $8 difference today.

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

“For families and other owners of more than one type of vehicle, the greatest fuel savings often comes from improving the efficiency of the less efficient car,” Soll added. “When fuel efficiency is expressed as gallons per 100 miles, similar to what is done in other countries, it becomes clear which combination of cars will save a family the most gas.� �

The authors recommend that consumer publications and car manufacturers list efficiency in terms of gallons per 10,000 miles driven. “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, Larrick said.