GM came out yesterday with the news that their Chevy Volt, due for sale late next year, will get 230 mpg.

To quote one of my favorite old-school Howard Stern soundbites, “whaaa whaaaa whaaaaaat?”

How exactly is Chevy of all carmakers going to effectively quadruple the mileage record for a standard-production vehicle? Is this just a massaging of the figures or could it actually be legit?

The EPA told Edmunds.com’s Green Car Advisor it hasn’t confirmed the 230 number, but that it “does applaud GM's commitment to designing and building the car of the future.” Hmm. That’s not very definitive sounding.

But it’s apropos, because there’s nothing definitive about the method apparently used to come up with the 230 number. In fact, it’s pretty arbitrary. Let’s start at the beginning.

The Volt is powered off an electrical charge, which comes from plugging it into a regular wall socket, that lasts for the first 40 miles. After that, its electricity is generated by a good old-fashioned internal combustion, gas-powered engine.

GM’s calculations – according to Volt project manager Tony Posawatz, as cited by Edmunds.com – don’t include whatever mpg equivalent the electric motor uses. (Mpgs for electric vehicles like the Volt are conversions, equivalents, computations of computations. They’re not the same as for standard-fuel vehicles, because their fuels are measured differently – you don’t have gallons of kilowatts, after all. But forget about all that, because GM isn’t even taking that into account for their 230 number. Yikes!)

This is how I imagine GM’s simple (disingenuous?) logic went (anything in parentheses is my editorial voice butting in on my pretend GM voice. Get it?): OK, here’s the mileage our motor gets when its electricity is gas-generated (let’s call it 40 mpg, based on reports that the gas tank is 8 gallons and it’s expected to be able to go 300 miles). Now, only a certain percentage of the time will a driver even need to use the gas-generated aspect, since the motor can go 40 miles on a plug-in charge (so the more often a driver goes fewer than 40 miles before plugging back in, the higher his mpg will be). And the Dept. of Transportation tells us the average American drives 29 miles a day. So let’s say our typical Volt driver will do this five days a week. Then on the sixth day, he rests; he only goes like 5 miles. And then on the seventh day he’s ready to really take off, so he’ll go like 80 miles. So let’s call it 230 mpg, since our typical Volt driver goes 230 miles (5 x 29 + 5 + 80) using one gallon of gas.

I know, kinda whacked. But it appears that’s the kind of approach going on here. And as arbitrary as it is, it still makes some sense, still seems honest in a way. What seems dishonest, though, is that they’re discounting whatever equivalent of mpg comes from charging via the plug-in. Once that’s included, it changes the calculations quite a bit. In the above scenario, for example, those six days when no gas is used currently amounts to an infinite mpg, but with a fixed, definable mpg equivalent, it would count against the 230 significantly.

Plus, there’s the emissions aspect. As I wrote when Chevy first announced the Volt, there are still emissions coming from somewhere; just not your tailpipe.