If the headline “Buy an S.U.V., Save the Planet,” over an article by Eric A. Morris of Freakonomics, sounds too good to be true—that’s because it is. It won’t help to trade your 46-mpg Prius for a 14-mpg Land Rover Range Rover Sport. If, however, you already have a 14-mpg Range Rover Sport, he suggests you exchange it for a 24-mpg 2009 Toyota RAV4 2WD—a move that will save you almost three gallons per 100 miles. If you’ve already got a RAV4, though, he seems to suggest that you might as well keep it—trading it for a Prius will save you only two gallons per hundred miles.

His valid point is that we can help the environment by “…simply getting people out of the worst gas guzzlers into moderately more efficient alternatives, even within the same fuel-hungry class.”

His other point is that the nonlinear relationship between mpg and fuel savings is “…a hard concept for us to get our brains around. Richard B. Larrick and Jack B. Soll, reporting in Science, found that only 1 percent of college students studied correctly perceived that an improvement from 14 to 24 mpg. saves considerably more fuel than an improvement from 24 to 46.” To give buyers a more meaningful rating, he suggests replacing the ubiquitous “mpg” window stickers with ones that indicate gallons per hundred miles.

Update: Re: Meredith Poor's comment below. It's called the rebound effect, which Morris addresses here. He says that recent research puts the rebound effect at 11%. That is, if you improve your gas milage by 40%, you'll drive 4.4% more miles.