To get the most out of a tank of gasoline, car shoppers should transpose a vehicle's "miles per gallon" number into "gallons per miles," suggests research by psychologists from Duke University's Fuqua School of Business.

As reported in the June 20 Science (Vol. 320, No. 5883), Richard P. Larrick, PhD, and Jack B. Soll, PhD, asked 322 college students and online survey participants to rank car improvements in miles per gallon. For example, they were asked to consider the impact on gas consumption and benefit to the environment if they switched from a car that gets 18 mpg to one that gets 28 mpg or switched from a car that gets 34 mpg to one that gets 50 mpg. The researchers found that most ranked improvements such as 34 mpg to 50 mpg-a savings of 94 gallons of gas per 10,000 miles-as more favorable than going from 18 mpg to 28 mpg-which saves 198 gallons per 10,000 miles. Yet when the improvements were described in terms of gallons used over a certain distance, most participants selected the more gas-conscious choice.

The findings show that posting a car's fuel efficiency in terms of gallons per miles may help consumers make more fuel-efficient vehicle selections during the car-purchase process and suggest that trading in an already-efficient car for a compact hybrid may not be as beneficial--for a buyer's wallet or for the environment--as getting a gas guzzler off the road, Larrick says.

"Buying the most efficient car is always the best, but there are surprisingly large savings in improving inefficient cars by even small mpg amounts," Larrick says. "Measuring a vehicle in terms of gallons per 10,000 miles makes gas savings more clear."

Test your ability to select the most fuel-efficient car trade-offs at www.fuqua.duke.edu/news/mpg/mpg.html.

-A. Novotney