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Automotive designers face three certainties in life: death, taxes and physics. The latter may be why more and more vehicles on the road today bear a striking resemblance to one another.

Designers can dream big, but when it comes time for a splashy concept car to transition to production, “we all have to abide by the same laws of physics. It doesn’t matter if we don’t like them,” says Nina Tortosa, General Motors aerodynamicist for the auto maker’s Voltec/E-Flex programs.

Aerodynamics long has played a role in increasing vehicle performance and fuel efficiency. But its role has taken on new importance as looming federal mileage requirements push auto makers to find more ways to stretch a gallon of gasoline further.

Most auto makers set a coefficient of drag (Cd) target early in a product-development program to guide engineers and designers toward that goal.

Princeton University defines aerodynamics as “the ratio of the drag on a body moving through air to the product of the velocity and the surface area of the body.” The lower the better.

The Chevrolet Volt extended-range electric vehicle serves as a prime example of how fluid dynamics ultimately determine the finished shape of a vehicle. With a Cd of 0.28, the Volt’s design offers less air resistance than any sedan in Chevrolet’s history, GM says.

But the car now plying U.S. roads looks remarkably different than the concept that bowed to great fanfare at the 2007 North American International Auto Show in Detroit. When it came time to ready Volt for production, Tortosa says many changes had to be made, noting the concept was just that.

“The initial changes from concept to production were matters of the platform,” she says. “The concept looked rear-wheel drive and the production version is front-wheel drive, so the proportions changed.”