General Motors jolted the automotive world yesterday by announcing that its upcoming electric-drive Chevy Volt will earn an EPA city mileage rating of 230 mpg. It's an astounding claim - one that would make the Volt more than four times as efficient as the Toyota Prius. And yet, even the federal government says it can't back up GM's math.

What's going on here?

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.

To attempt to solve that problem, the EPA has been developing a new testing process for EREVs. But they haven't finalized the process - and GM may have been premature in using it to arrive at their jaw-dropping number.

The EPA, in fact, won't back up GM's number. Instead, the agency released a statement reading, "EPA has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by GM," though they added, "EPA does applaud GM's commitment to designing and building the car of the future."

The EPA's tentative EREV testing process won't actually measure gasoline usage. Instead, it rates vehicles in kilowatt hours per 100 miles, then converts that measurement to miles per gallon. Effectively, the testing procedure doesn't give an mpg rating. It merely shows that a vehicle will use energy that equates to a certain mpg rating.

To illustrate this point, Nissan quickly followed GM's announcement with its own, claiming the upcoming 2010 Nissan Leaf electric car will earn a 367 mpg EPA rating. The rules, it seems, can generate a miles-per-gallon rating for a car that doesn't even use gasoline.

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.

But that explanation is a mouthful, and not nearly as easy to market as 230 mpg.

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