General Motors calls the Chevrolet Volt an extended-range electric vehicle. Essentially 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 engine, how did the EPA get that figure?
Popular Science calls it a "draft methodology." That's a quick way of saying the EPA is developing a few assumptions to populate a new "duty cycle" for the Volt. The duty cycle is the usage profile the agency uses when determining the city and highway mileage numbers to put on a new car's window sticker. The latest EPA cycle, set in 2006, accounts for actual driving conditions, such as high speed, aggressive driving, use of air conditioning, and cold temperature operation.
As John Voelcker from GreenCarReports.com points out, GM says the Volt can travel for the first 40 miles on battery power alone. That means, if you never drive more than 40 miles a day, your mileage is technically "infinity." Of course, that isn't quite accurate over longer distances. So the EPA likely adopted a test cycle that involves driving the Volt until the battery is discharged, and then for a further distance using gasoline power.
Read more about it HERE
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