UPDATE: GM confirms that the 2011 Chevrolet Volt will get 230 mpg city, and a composite fuel economy (city and highway) of more than 100 mpg according to draft regulations being developed at the Environmental Protection Agency (EPA).

If the rumors are correct, this morning General Motors CEO Fritz Henderson will finally reveal what the company's mysterious '230' ad campaign was about. It appears to be the official mileage rating for GM's upcoming 2011 Chevrolet Volt extended-range electric car.

A number like that seems outlandish, absurd. How can the US Environmental Protection Agency possibly measure fuel consumption that low? The answer, it turns out, is all in the assumptions.

40 miles, no gasoline

The Volt, remember, stores energy in both a gasoline tank and a battery pack. And it will always prioritize using electricity from the battery to power itself before it ever switches on the gas engine. Unlike a conventional hybrid car, though, the battery pack is usually recharged by plugging the Volt into a wall socket.

But the Volt's 16-kilowatt-hour battery pack only gives it 40 miles of electric range. To eliminate "range anxiety," after that, the Volt switches on its engine to run a generator that provides power to its electric motor. That gives another 300-plus miles of range.

So depending on how many of the Volt's miles are run on grid power, and how many by burning gasoline in the engine to generate its own electricity.

GM often cites the statistic that more than 70 percent of all US vehicles travel less than 40 miles a day. If your usage falls within that level, your Volt would never turn on its engine--and never use a drop of gasoline. That's gas mileage of, well, infinity.

On the other hand, if you drive a Volt 140 miles every single day, still recharging it at night, it would travel 40 miles on grid power and 100 miles on gasoline. If the car gets 50 miles per gallon with the engine on, that's two gallons burned, 140 miles total, or 70 mpg.

The more daily miles over that first 40, the higher the proportion of gasoline burned--and the lower the overall mileage.

What to assume about usage?

Which leads to the big question: What assumptions should the EPA make in its emissions and gas-mileage tests about how the Volt is used (also known as the car's "duty cycle")?

For decades, gasoline cars (and hybrids) have been testing using two cycles: city and highway. That gives us the two quoted EPA mileage ratings, and the EPA also calculates a "blended" number for overall usage. The distance driven doesn't really matter.

But for the Volt, mileage assumptions become much more political.  If the EPA tests a Volt over a cycle of less than 40 miles, it will never burn any gasoline, and it'll get that "infinite" mileage. The daily distance matters much more for the Volt than for a gas engined car.

The answer appears to be the EPA has adopted a cycle described by GM-Volt.com, among others, that assumes the Volt is driven until the battery is discharged--and then slightly more on gasoline power.

Your mileage may vary

A similar test routine proposed by Mike Duoba at Argonne National Laboratories repeatedly drives the car on four EPA highway test cycles until the battery is discharged, then drives one city cycle--totaling 51 miles. (The EPA city cycle is roughly 11 miles, the highway cycle about 10 miles.)

If the engine runs for 11 miles at 50 mpg, that will use 0.22 gallons of gasoline. But that amount is used over a total travel distance of 51 miles, which works out to 232 mpg. Sounds like 230 mpg to us!

In the end, we can only return to that old saying: Your mileage may vary. And in the case of the 2011 Chevrolet Volt, it may vary a whole lot more than in any car running purely on gasoline. But, hey, no one ever said being green meant easy math ....

2010 chevrolet volt 019

2010 chevrolet volt 019

[GM-Volt via TheCollegeDriver]