The problem with cars like the Volt and Leaf is that they don't use fuel in a traditional manner, and thus cannot be directly compared with ordinary vehicles. The Environmental Protection Agency, the government agency that oversees fuel-economy ratings for automobiles, is wrestling with how to rate plug-in vehicles. And if we're being honest, we haven't the faintest idea what they're going to come up with.

As The New York Times reports, the EPA's problem is twofold: first, it must figure out a way to measure the relative efficiency of cars like the Volt; and second, it must determine how to present that information in a manner that consumers can easily comprehend. According to The Times:

The EPA has proposed changing the labels on all new cars, including the possibility of assigning an overall letter grade, in part to address the issue of electric and hybrid vehicles. But the proposal would not take effect until the automakers’ 2012 models. In the meantime, [the agency says it is] trying to determine “the appropriate information that will go on the 2011 model year label.”

The challenge is a big one. American consumers have no benchmark for plug-in vehicles, whether through direct comparison or historical reference. You can count the world's successful alternative-powertrain vehicles on one hand, and most of them never saw widespread use. With cars like the Volt, a fuel-economy rating, or something like one, is key to the public's understanding of the car's appeal. It's not far off to say that sales could be affected by the EPA's decision.

This also isn't a new issue. One year ago, General Motors famously announced that the Volt would be capable of achieving 230 mpg, a figure that was scoffed at by pundits for being unrealistic. Enthusiast publications like Motor Trend have since reported observed triple-digit fuel mileage, adding the caveat that economy depends entirely on how the car is used. And the Tesla Roadster, an all-electric sports car built in California and on sale since 2008, isn't currently shown in the EPA's Fueleconomy.gov database, despite Tesla's claims that the car achieves the equivalent of hundreds of miles per gallon.