The truth comes out - eventually.

A couple of days ago, I questioned the claims of General Motors over their 230 miles-per-gallon Volt vehicle, which runs on a battery and gasoline.

As often happens, once something is exposed to light, the blemishes are exposed, and the people who make the claims are forced to admit to the actual facts, which is something that marketing people abhor.

The Volt is one of several so-called Extended-Range Electric Vehicles, or EREVs, in development. An EREV functions as an electric car until its batteries are depleted to a certain level; then it starts a small gasoline engine. That engine, however, doesn't drive the wheels - it merely acts as a generator to recharge the batteries. The Volt, GM says, can travel about 40 miles at any speed before its onboard generator kicks in. That number is significant, because Department of Transportation figures show that most Americans drive less than 40 miles per day. For most of us, owning a Chevy Volt could mean rarely ever using gasoline. That possibility, however, makes measuring the car's predicted fuel economy a tricky process.

So what kind of mileage can a Volt driver actually expect?

That depends entirely on how much they drive. GM claims the Volt has a 300-mile range after the gasoline engine ignites, but the company hasn't released what the size of the Volt's gas tank will be. Engineers have said it could be as small as 8.5 gallons - and 300 miles on 8.5 gallons means the car might be no more efficient than 35 mpg. Your mileage, then, will vary greatly based on how far you drive. For 40 miles, the Volt uses no gas. After that, it's probably going to net about 35 mpg. So if you commute 40 miles or less per day, you could expect an infinite number of miles per gallon. For every mile you drive over 40, the number drops precipitously, stabilizing at around 35 mpg as the needle drifts toward empty.

That's some complicated math, but I think if you factor-in the electricity usage and its accompanying bill, your mileage may vary - significantly. I'm guessing that Volt owners will be disappointed to find that they're not getting much better mileage than their neighbors' Ford Focus on their commute to work, and the added boost to their electric bill will make them even more angry.

That's just what the auto makers need - more angry consumers.