Researchers say car manufacturers should advertise the fuel efficiency of vehicles

Car manufacturers should change the way they advertise the fuel efficiency of their vehicles, to make the environmental impact of buying a new car easier to understand, researchers said yesterday.

Turning a car's fuel efficiency on its head by stating it in "gallons per mile", instead of "miles per gallon" helps potential customers choose the greenest option when they upgrade, said the team at Duke University in North Carolina.

In The MPG Illusion, published in the US journal Science yesterday, Richard Larrick and Jack Soll at the university's Fuqua School of Business describe how the use of miles per gallon is misleading and causes people to grossly misjudge the environmental impact of upgrading to a new car.

The two management professors stumbled across the problem while working out the true fuel efficiency of different cars in a car-sharing scheme. They found people often believed - mistakenly - that a 10mpg improvement in fuel efficiency always corresponded to the same fuel saving.

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

Likewise, replacing a car that does 10mpg with one that appears only slightly more efficient at 11mpg saves as much fuel as upgrading from a 33mpg car to a 50mpg car.

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