By John DeFore

Quick question: Jack trades in his 15 mpg truck for a car getting 25 mpg, while Jill upgrades her 25 mpg sedan for a 35 mpg compact. Assuming they drive the same amount, who’s going to save the most money over the coming year? (Ignore for the moment that Jill’s commute has a better bottom line, financially and environmentally.)

Instinctively, most of us would say they’d tie. After all, each will be getting an extra ten miles out of every gallon. But take a moment to do the math, and you realize that Jack will save more than twice as much: If both drive ten thousand miles next year, he’ll save 267 gallons to her 114.

That error in reasoning, according to professors at Duke University’s Fuqua School of Business Richard Larrick and Jack Soll, is due to the fact that fuel consumption doesn’t fall at an even rate when efficiency improves. In an article in this week’s Science magazine, the researchers suggest that misunderstandings could be avoided by talking about efficiency using more intuitive language. Instead of ranking cars in terms of miles per gallon, we should cite the number of gallons per mile. (Actually, to avoid small fractions, they suggest listing gallons per 10,000 miles.)

If automakers used that figure, SUV shoppers could see just how big a difference every extra bit of added economy makes at the low end of the fuel-efficiency spectrum, while those of us with very efficient conventional cars would feel less pressure to ditch them and spend tens of thousands on hybrids that don’t perform that much better.

As Soll said in a statement about the report: “We believe that everyone should try to be as fuel efficient as possible. For some people, that may mean driving the most efficient car available, such as a small hybrid car, but for others it may mean finding the most efficient option possible within their chosen class of car. There are significant savings to be had by improving efficiency by even two or three miles per gallon on inefficient cars, but because we communicate in miles per gallon, that savings is not immediately evident to consumers.”

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