The MPGe rating applies to more than just cars that run on battery power. It's also intended to help make accurate comparisons of cubic feet of compressed natural gas and kilograms of hydrogen to the more familiar gasoline.
Beginning with the 2013 model year it will be mandatory to show the MPGe number on new car and light truck information labels for what the EPA terms "advanced technology vehicles," including electric vehicles, electric-gasoline hybrids, compressed natural gas-powered vehicles and fuel cell vehicles [source: EPA].
Conventionally fueled vehicles will continue to carry the standard "mpg" listing, and all new car and light truck window stickers will list a bunch of text and pictorial information about each vehicle's fuel efficiency and emissions. In addition, auto shoppers equipped with QR code-capable smartphones can scan even more information from a QR symbol printed on the new vehicle's window label [source: EPA].
And for the record, the methods for calculating MPGe for window labels and for calculating Corporate Average Fuel Economy (CAFE) numbers are two different formulas. One of the critiques of MPGe is that it uses a "tank-to-wheel" approach instead of "well-to-wheels" -- the latter takes into account how the energy was produced "upstream," at say, an oil refinery or coal-fired power plant [source: Weissler].
While the new measurement might not satisfy everybody, there's no denying that it marks a major, ahem, milestone in the advancement of alternative fuel vehicles.