An Environmental Protection Agency (EPA) proposal for computing a fuel efficiency rating for hybrid-electric and all-electric vehicles using a "miles per gallon" scale is in the works. But according to USA Today, an Israeli company that specializes in self-charging hybrid-electric propulsion systems for cars wants the government to consider a new three-pronged rating for fuel efficiency.

The EPA draft proposal would be a rating buyers could use to compare various vehicles. But Israeli company ETV Motors, which specializes in self-charging hybrid-electric propulsion systems for cars, has suggested a three-pronged rating for fuel efficiency: One number would show how far the car can go on a single plug-in battery charge, a second number would show how energy intensive the battery is, and a third for how much gasoline it consumes to drive a generator or the wheels when the battery runs out.

ETV Motors CEO Dror Ben-David says a single-number rating might be misleading. But a senior engineer for the Union of Concerned Scientists, says the government could use various methods to set a standard for plug-in electrics, but no option is simple.

The engineer, Jim Kliesch, says they could measure the car's greenhouse gas emissions. Using an mpg rating formula may be misleading, he says, because people may not get that efficiency in the real world.

"It's really critical that the test procedure that's decided on accurately reflects what people are going to experience," Kliesch said.

Using an EPA draft proposal, General Motors has forecast that its plug-in Chevrolet Volt electric car will be rated at 230 mpg, though the EPA cannot confirm that. Nissan, using different numbers from the Department of Energy, claims its Leaf plug-in car will get the equivalent of 367 mpg.