DETROIT (CNNMoney.com) -- The Chevrolet Volt, GM's electric car that's expected to go on sale in late 2010, is projected to get an estimated 230 miles per gallon, the automaker announced Tuesday.

That exceptionally high government mileage rating could give the Volt a major boost. For the first time, car buyers will easily be able to compare electric cars with ordinary gas-powered cars.

"Having a car that gets triple-digit fuel economy can and will be a game changer for us," said GM CEO Fritz Henderson.

Determining fuel economy for an electric car is a tricky matter. While General Motors (GM, Fortune 500) has been working with the Environmental Protection Agency for years on the issue, the agency says it has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by GM.

Basically, you will be able to drive the Volt for about 40 miles using the lithium-ion batteries. For those driving less than that, gas mileage is essentially unlimited. It is only after 40 miles that the Volt will start using gas.

"Most Volt drivers will operate on a daily basis without having to use a single drop of gas," said Henderson, saying that three out of four drivers travel 40 miles or less a day.

Fuel economy for hybrid vehicles like the Toyota Prius is displayed in the same way as it is for any other gasoline-powered vehicle. It gets 46 mpg, for example, versus 19 mpg for a V-6 Ford Mustang.

That standard works because all the energy used by the Prius ultimately comes from burning gasoline. The Prius just uses that energy more efficiently than other cars do.

The Chevrolet Volt, on other hand, runs on electricity that comes from two sources -- a battery as well as a gasoline engine.

When gasoline is providing the power, the Volt might get as much as 50 mpg. But that mpg figure would not take into account that the car has already gone 40 miles with no gas at all.

So let's say the car is driven 50 miles in a day. For the first 40 miles, no gas is used and during the last 10 miles, 0.2 gallons are used. That's the equivalent of 250 miles per gallon. But, if the driver continues on to 80 miles, total fuel economy would drop to about 100 mpg. And if the driver goes 300 miles, the fuel economy would be just 62.5 mpg