55 mpg is the new 40

Government uses at least two different sets of numbers to describe vehicle fuel economy, one based on a 1975 law passed by Congress, which measures fuel economy in a lab, and one recently refined by the Environmental Protection Agency, which attempts to estimate fuel economy in real-world driving conditions, according to Roland Hwang, the transportation program director for the Natural Resources Defense Council.

The stickers on car windows, like the one that says the Prius gets 50 mpg? Those are the EPA's real-world estimates.

The 2025 fleetwide average of 55 mpg that the Obama Administration and automakers just announced? That's a figure based on a laboratory certification.

The difference between the lab numbers and the real world numbers is about 20% (possibly as much as 30%), according to Hilary Sinnamon, a clean air consultant for Environmental Defense Fund.

The Prius actually gets about 70 mpg as measured in the lab, Hwang said. And he predicted the real-world average fuel economy in the year 2025 will be closer to 40 mpg. John O'Dell, a senior editor and green car advisor at Edmunds.com, said the figure might be closer to 38 mpg.

Lest we scoff at the grand pronouncements Obama and others have made about the environmental benefits of such a fuel economy upgrade, 40 mpg is still nearly double today's average of 22 mpg.

"It's a major step," Hwang said. "It's the biggest thing the president can do to simultaneously reduce oil dependency, save consumers at the pump and reduce carbon pollution."