Here's a challenge for the Environmental Protection Agency: how do you rate the fuel economy of cars that don't burn gasoline, like the battery powered Nissan Leaf? You start by making the assumption that a gallon of gas has the same equivalent energy to 33.7 kW-hrs worth of batteries, then you measure the actual battery capacity of the electric car and calculate its range. The EPA has done that with the Nissan Leaf, and has given it an "MPG Equivalent" rating of 99 miles per gallon.
The Leaf was given an estimated average range of 73 miles on its 24 kW-hr battery pack, so the EPA calculated that a Leaf equipped with 33.7 kW-hr batteries would be able to travel 99 miles on a single charge, hence the rating. The EPA is still working on a rating for the Chevrolet Volt extended range electric vehicle (or serial hybrid, if you prefer), since they must combine a range on battery power plus a range supplied by the gasoline powered generator.