When a gas guzzler can be good for the environment

It's a critical question these days on any car lot: How many miles per gallon does a vehicle get?

But a new report out in the journal Science today says that's not the right question. We really should be asking how many gallons per mile the car gets.

That's because people often believe the amount of fuel a car uses decreases at an even rate as miles per gallons improve. While that feels intuitive, it's not as easy as that, says Duke University researchers Richard P. Larrick and Jack B. Soll.

Can a hybrid SUV save the environment?

Consider: Swapping a 34 MPG car with a 50 MPG car would save about 94 gallons for every 10,000 miles driven. But switching a 18 MPG car with a 28 MPG one saves 198 gallons.

It turns out small gains in the biggest gas guzzlers might be helping the environment more than people think.

The two researchers ran a series of experiments with people who were presented with a series of car choices and asked to pick which ones would result in the greatest gains in fuel efficiency. Participants were not able to easily identify the correct choice when given miles per gallon. But they chose correctly when presented with gallons per mile.

That's not to say everyone should go out and buy a SUV hybrid.

Of course, the environment would benefit the most if everyone was driving Priuses, the researchers note. But it does indicate U.S. drivers may be underestimating the real value of getting the worst gas-guzzling cars off the road.

Larrick and Soll want car manufacturers and consumer magazines to start posting and talking about fuel efficiency in gallons per mile so consumers can make better decisions about car purchases and environmental impacts.

"For family and other owners of more than one type of vehicle, the greatest fuel savings often comes from improving the efficiency of the less efficient car,'' Soll said. "When fuel efficiency is expressed as gallons per 100 miles, it becomes clear which combination of cars will save a family the most gas."