[Update: The EPA issued a statement to the folks at Edmunds stepping back from GM's mileage claim: "The EPA has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by GM. EPA does applaud GM's commitment to designing and building the car of the future - an American-made car that will save families money, significantly reduce our dependence on foreign oil and create good-paying American jobs."]
General Motors calls the Chevrolet Volt an extended-range electric vehicle. That's because the only motive force comes from the electric motor; the gas engine only charges the batteries. In a press conference earlier today, GM's CEO Fritz Henderson said the Volt will have a city mileage figure of 230 miles per gallon--almost five times more efficient than a Prius. But considering the uniqueness of the Volt's powertrain, how did the EPA get that figure?