[ad#post_ad]As the Volt story winds towards the day of first public sale, now less than two months away, few secrets about the car remain. After the pricing announcement, one of the most elusive pieces of information remaining is what how many miles per gallon the Volt will achieve once in charge-sustaining mode.

GM argues, rightly so, that figure isn't very important because it doesn't take into account the typically utility of the car. In reality, drivers will plug in their cars nightly, and 75% will only drive within the pure EV range of 40 miles. Longer drives will be rare. Thus miles per gallon over time is most relevant. As is typical of human nature though, we still strive to compare the Volt to cars on the street today. Once the Volt's battery is depleted and the car continues on long drives it will operate on gas. People would like to know how many miles per gallon it will get in once it enters that mode.

When the Volt concept was first unveiled in January 2007, GM was very forthright about its goal for this serial mode efficiency; 50 MPG was announced. That could be achieved, according to math models, using a 3 cylinder turbocharged 1 liter engine. For production, however, GM chose a larger displacement normally aspirated 4 cylinder 1.4 L engine which was cheaper to produce and more widely available. Furthermore, since it is also well known that a serial hybrid design isn't as efficient as a parallel hybrid design, it is uncertain the Volt can achieve 50 MPG in charge sustaining mode, though engineers developing the car said recently that was "still the bogey."

Yesterday we saw a pretty cool test drive video shot by AOL Autos that happened to show for a 16.1 mile charge-sustaining mode trip the car appeared to use .59 gallons of gas suggesting the car may have an mpg less than 30.

GM for its part has said it does not plan to announce charge sustaining MPG instead expecting people would find this out on their own once prolonged test dives by reporters and owners take place.   The final EPA label will include the utility factor and daily charging. The GM-Volt report coming from the AOL video and now having propagated to Edmunds , Autoblog , and others illustrates the risk of this strategy.

"These numbers are completely out of context and irrelevant," says GM spokesperson Rob Peterson. "As you can tell from the video itself, the AOL Translogic team ran a battery of aggressive tests with the vehicle including extensive use of mountain mode, time trials (0-60), (and) aggressive driving maneuvers."

"Your...calculation did not account for the significant amount time the vehicle spent operating while waiting for AOL Translogic film crew to get into place," he added.

"I'm hard stretched to think of ANY real world conditions under which a Volt owner could simulate the conditions this particular vehicle was put under," he said.

I once had the chance to drive a 67% Volt prototype for about an hour on a half mile test track. I was told at the time the efficiency calculation algorithm still wasn't refined, nor was the car, but after resetting the meter once in charge-sustaining mode found I could bring the MPG close to the 40 mark with reasonable driving style.

In fact, I suspect the Volt is approaching the biblical number 40 from along several axes; EV range, price, and CS fuel efficiency. But that's just my guess, because as you known GM won't say.