General Motors announced on Tuesday that its upcoming Chevrolet Volt extended-range electric vehicle will be given a city fuel economy rating of at least 230 miles per gallon by the EPA, nearly five times more efficient than the current Toyota Prius.

The Volt, which runs purely on electricity for the first 40 miles of driving, carries a small internal combustion engine on board to generate electricity after the battery has drained. The EPA is currently developing a special methodology to calculate fuel efficiency for vehicles that work in this fashion which averages the results of test cycles run separately in electric and internal combustion modes.

The Volt’s highway and combined fuel economy ratings have not been calculated yet, but during the press conference, GM’s C.E.O. Fritz Henderson said that that the combined figure would be in the triple digits, meaning at least 100 mpg.

The 4-passenger hatchback is expected to go on sale in late 2010 for approximately $40,000.