[ad#post_ad]GM has talked tirelessly abut how aerodynamic the Volt is, how its shape was born from the wind tunnel, and how important aerodynamics are for the car to achieve its 40 miles all electric range.

Yet for all that talk, the company has never released any official figure for the vehicle's coefficient of drag (CD). This is the numerical measurement that indicates how slippery a car is, and unlikely to be slowed by wind resistance.

Bob Boniface who is chief of Voltec design finally provided us the details that were obtained when GM measured the Volt and its competitors on its own wind tunnel.

"We had the comparably-equipped 2010 Prius with 17 inch wheels, and the new Insight," he said.

"The Prius came in at .30," said Boniface. "That was a number that was verified in our tunnel, in Chrysler's tunnel, and in Ford's tunnel."

"The Insight was 0.32, and the Volt was .28," he said.

"I'd like to test the Volt in the same tunnel where Toyota got their 0.25 value," he teased.

He expanded:

This number is for the Volt IVer which is representative of our production car. We were resistant to give out the number earlier for two reasons. One we wanted to wait until we tested our IVer. Also, we didn't want to report it out until we saw where the competition was because we know that those numbers depend on how the tunnels are calibrated.

If I quoted 0.28 a year ago people would have said 'aha' the competition got 0.25. But its really all relative to what tunnel it was tested on.

The base Prius with the smaller wheels may come in lower, but we don't offer 15 inch wheels.

According to GM's aerodynamics engineer Nina Tortosa, the old method of testing yielded .19, but with current testing methodology it would equate to .21.

"But that vehicle didn't have a rollover and frontal impact structure of today's standards," said Boniface. "You could not sell that car today. That's not to say we wont get to .21 again with another car. We always try to get drag down but we have to protect our styling flexibility."

The current base Corvette is the most slippery and its a 0.29, slightly less aerodynamic than the Volt.