If you’re looking for a new car to maximize your fuel savings, don’t fall into the trap of merely comparing mpg ratings. That message comes from Duke University professors in a recent article in the journal Science.

“Miles per gallon is misleading and can play tricks on our intuition,” says Jack Soll of Duke’s business school. He and co-author Richard Larrick point out that fuel consumption doesn’t decline at an even rate as fuel efficiency climbs. For example, most people think that an improvement from 34 to 50 mpg saves more gas in a year than an improvement from 18 to 28 mpg. After all, it represents a difference of 16 mpg vs. 10 mpg, right? In fact, over 10,000 miles, going from 34 to 50 mpg saves only 94 gallons. But going from 18 to 28 mpg saves a whopping 198 gallons.

“For owners of more than one type of vehicle, the greatest fuel savings often come from improving the efficiency of the less-efficient car,” notes Soll. To emphasize this “law of diminishing returns,” Soll and Larrick recommend that the government report fuel efficiency as gallons per mile.

Actually, the U.S. Environmental Protection Agency (EPA) already reports fuel efficiency in a similar way—gallons per 100 miles—at www.fueleconomy.gov. On the window sticker of every new car, the EPA also lists annual fuel cost—a surrogate yardstick for Larrick and Soll’s gallons per mile. Moreover, by using fuel cost rather than mpg to compare vehicles, the differences become obvious in the examples above. With gas now below $2 per gallon, going from an 18-mpg to a 28-mpg vehicle would save $396 over 10,000 miles, while going from 34 to 50 mpg would save only $188.