VSL:SCIENCE // How to think about greener cars

Down in North Carolina, Duke management professors Jack Soll and Richard Larrick passed their morning commutes pondering the vagaries of fuel efficiency, and came up with the following thought experiment: What saves more fuel: (A) Replacing a 10mpg Escalade with a 20mpg crossover? Or (B) replacing a 25mpg Accord with a 50mpg Prius?

Counterintuitively, the correct answer is (A). Why? Because over the course of 10,000 miles, a 10mpg SUV will use 1,000 gallons while the 20mpg crossover will use 500 gallons — resulting in a 500-gallon savings. On the other hand, an Accord will burn 400 gallons and the Prius 200, for a net conservation of only 200 gallons.

If you guessed wrong, you’re in good company: After all, mpg is how automotive fuel efficiency has always been presented to us, and converting efficiency into consumption (as we have above) is a convoluted process. But Soll and Larrick found that when they expressed the same question in tems of gallons per miles, their subjects immediately realized that replacing the 10-gallon-per-hundred-mile megamonster with a 5-gallon-per-hundred-mile mini-monster brought the bigger gain. According to their study, the EPA should replace mpg ratings with a gallons-per-mile metric — such as X gallons per 10,000 miles — which would give consumers a better sense of their annual consumption. Definitely an idea worth discussing during your next car pool.

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