The U.S. Environmental Protection Agency will soon draft a new regulation for the way it calculates fuel efficiency for electric cars, potentially deflating the stratospheric fuel economy numbers trumpeted by automakers in recent months.

In August 2009, GM announced that its new 2011 Chevy Volt hybrid would achieve a stratospheric 230 "miles per gallon," using a calculation that took into account how the vehicle would fare given its sometimes-battery, sometimes-gasoline engine power source. Nissan responded with its own number for its new Leaf electric car, 367 MPG, setting off something of an arms race for efficiency. Without actual guidelines from the EPA, however, the public will be left wondering where these numbers come from -- and whether they're believable. The EPA is unlikely to weigh in until after GM and Nissan launch their new vehicles, which has left the automakers to craft their own efficiency ratings.

"They're not going to have actual rules until model year 2012, which is a little late for the Volt," said Pete Savagian, GM's engineering director for hybrid powertrains. As far as how GM will tell consumers what sort of fuel economy they'll see on the Volt? "We're on our recognizance. I don't exactly know. We'll have to be real careful."

This ambiguity isn't the best for consumers. GM's calculation takes into account only the gasoline used, while Nissan's number was merely an equivalent since its Leaf only runs on electricity. Compounding the problem is that consumers aren't used to fuel economy figures for anything other than miles per gallon.

GM came up with the Volt's number using a guideline from the Society of Automotive Engineers, one that ran the car through a city driving simulation. GM did the same simple calculation for fuel economy that we use for traditional vehicle, dividing the total distance travelled by the amount of gasoline burned. But for a good portion of the test, the Volt was operating on battery power alone, having started the test with a full battery. This effectively adds the 40 miles of battery range to the distance travelled, hence the big mileage number.

"230 [MPG] is not a equivalent," said GM's Savagian. "It's just the gasoline. It does not include the electricity consumed. It only included the amount of gasoline used and the number of miles driven."

The fact that the Volt engine burns gasoline will come into play when the California Air Resources Board (CARB) determines its rating, as well. The Volt will likely not be labeled as a zero-emission vehicle (ZEV), but rather as a partial zero-emission vehicle (AT-PZEV) for using gasoline after the battery power is depleted.

For now, the company's focus on "230" has faded. GM pulled the plug on many of its media efforts that featured the infamous number. In August, the company had launched a television campaign that dovetailed with social media efforts on YouTube, Facebook, Flickr and a blog. All of those accounts have since been discontinued. The domain name "whatis230.com" is still registered to GM, but now redirects to a page about the Volt on Chevrolet.com.

No Current EPA Standard

Confusion over the issue isn't so much political as it is chemical. Gallons and kilowatts aren't easily compared. But political forces are also at play. Right now Nissan, GM and a host of other manufacturers continue to provide their recommendations to the EPA on how the new ratings should be calculated.

Nissan is pulling for a new standard that looks at electricity use not in terms of a miles-per-gallon equivalent (although the company admits it did do this with its 367 MPG claim), but something that takes into account grams of C02 or kilowatts.

"We believe it's probably going to be a new definition," said Tracy Woodard, Nissan's director of government affairs. "I don't know whether you're going to get into grams per mile or kilowatt hours per mile, but I think you're going to see a shift away from miles per gallon."

Last weekend, the Wall Street Journal reported that the EPA was "rethinking" the way they thought about fuel economy, but the agency disputed that claim.

"I think that threw some people off," said an EPA spokesperson in an interview with AOL Autos. "We're not 'rethinking' because we're still thinking. In August, we simply said we're developing it. We haven't yet published the new rule."

"We are waiting for the EPA to tell us how we're going to formulate that," said Nissan's Woodard. "We're all just kind of waiting."