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.

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.

Calculating the gas mileage of the Volt is tricky. You get 40 miles of driving gas-free on the electric charge. Beyond that the Volt gets 50 mpg. So the actual mpg is:

mpg=Miles Driven/((Miles Driven-40)/50mpg)

Simplifying, the equation for calculating the mpg for the Volt is:

mpg=(50 * miles driven)/(miles driven-40).

This holds for trips greater than 40 miles. At 40 miles driven or less, the mpg is infinite since no gas is being used. Beyond 40 miles, the mpg decreases the more miles driven. For a trip of 50 miles, the Volt gets 250 miles per gallon. But, for a trip of 200 miles, the Volt gets 62.5 mpg. Based on the EPA decision to rate the Volt at 230 mpg, they are assuming a trip of 51.11 miles.

Left unanswered: what is the EPA mppc (miles per pound of coal) for the Volt when running off the electric engine?

The Chevy web-site notes that the EPA will rate the Volt as using 25 KwH of electricity/100 miles driven for an average price of $.75 to $2.50 per 100 miles driven on electricity. At $2.50 per gallon and 50 mpg, the same 100 miles would cost the driver $5 driving on gas.