The media is having a field day with the Chevy Volt, suddenly Detroit's most wanted. General Motors CEO Fritz Henderson claims that the car's "triple-digit" 230-mpg fuel economy is a "game changer." True? Let's take a closer look.

GM did not pull that 230-mpg figure out of its corporate hat; it used the EPA's official methodology for plug-in hybrids in city driving. (The highway protocol doesn't exist yet.) The problem is that this car runs only on electric power, never directly on gasoline, so talking about "mpg" at all is kind of a stretch.

I was on Neil Cavuto's Fox Business show last week talking about the Volt, and he — like most Americans — needs help understanding how the Volt works. Yes, the Volt has a gasoline engine, but it's only there to act as a generator to supply the electric motor. You plug it in, charge up, drive 40 miles on battery power, then the gas engine keeps you on the road by generating electricity. Does that sound like a process that can easily be measured by a mpg standard?

Think of it this way. The Volt travels the first 40 miles gas free. So if it got 40 mpg with the engine running (it actually does better than that) then in the first 80 miles you could say it got 80 mpg, because that's how far it traveled before burning up a gallon of gas.

Your results will depend a lot on how you drive. As Mark Chu-Carroll points out on Science Blogs, if your commute is 12 miles each way (like his wife's) then your gas mileage is essentially infinite — you'll always run on battery power. Chu-Carroll's own commute is 48 miles round trip, so he'd end up at 288 mpg. If his friend, with a 90-mile commute, drove a Volt, why then he'd get 90 miles per gallon.

GM is all over the 230-mpg thing, creating a smiley-face logo it put on T-shirts and bumper stickers. It served its purpose: The media went on a Volt feeding-frenzy. I've never seen such coverage of a car that's likely to sell only 40,000 units in its first two years (because GM won't make more than that).

There's a reason all the plug-in conversions of the Toyota Prius say "100 MPG" in huge graphics on their sides. The math is the same: The Prius gets 50 mpg, so if the first 50 is on batteries, then it takes 100 miles to go through a gallon of gas. In a straightforward plug-in hybrid, the gas engine drives the wheels after the battery range is exhausted. Ford, GM, Toyota, a whole bunch of companies are coming out with cars like that. But the Volt is in a category by itself.