How many miles will an electric car go on a gallon of gasoline? This is not a trick question. Federal law requires all new cars sold in the U.S. to feature a window sticker that lists fuel efficiency as measured in miles per gallon. Electric cars—which, of course, use no gasoline—are not exempt.

This was the dilemma faced by the Environmental Protection Agency as it redesigned the window sticker. The new designs calculate the efficiency of electric and plug-in hybrid vehicles according to a figure called miles per gallon equivalent—a rough estimation of how much energy the vehicles consume. The EPA stickers feature a flood of new information, including ratings of greenhouse gas and smog emissions.

Still, many environmentalists howled at the choice of sticker design. The New York Times quoted Dan Becker, director of the Safe Climate Campaign, as saying that "the Obama administration has dashed consumers’ hopes for clear information to make educated choices about which cars are really clean." He was referring to an alternate sticker design—one not chosen by the EPA—that was dominated by a single letter grade. In this system, electric vehicles would get an A+, while the worst gas guzzlers would be saddled with a D (if it’s approved for use on the road, it’s not a failure, the thinking goes).

The problem with this system is that it blurs the distinction between similar vehicles. Want a mid-size sedan? Under the letter-grade system you’d be choosing between cars that get a B- and cars that get a B. (As we noted last year, 88 percent of all new vehicles would score somewhere between a B and a C.) In effect, the letter-grade system would tell consumers that electric vehicles are better than plug-in hybrids, which are better than regular hybrids, and so on down to the Ferrari/Suburban pit of pollution. This, the EPA wisely decided, is of little use to the average car-buyer.

Even though the new stickers are cluttered with numbers, they end up giving consumers a clearer idea of the true costs of ownership. The traditional miles-per-gallon metric is deeply flawed, as it doesn’t provide easily scalable view of the differences in gasoline consumption among vehicles. For example: The two miles per gallon separation between SUVs getting 16 mpg and 18 mpg will lead to a larger difference in total gasoline consumed than the 15 miles per gallon spread between 40 mpg and 55 mpg vehicles.

The EPA accounts for this with a neat metric in bold, just to the right of the standard mpg figures: The total cost of gasoline, summed over five years. More than anything else, this is what the typical U.S. consumer wants to know—how much gas am I going to have to buy? By this metric, electric and plug-in hybrids look darn cheap. Environmentalists should be cheering.

Image: The new window sticker for a plug-in hybrid electric vehicle, courtesy of the Environmental Protection Agency