Aerodynamic Design and Mileage

Resistance to Reducing Resistance

Improving the way a vehicle moves through air is simultaneously the least expensive and most effective way for carmakers to increase fuel efficiency. More than half of the energy required to move a car traveling at highway speeds is spent on aerodynamic drag. Automobile designers have dozens of techniques for making their vehicles slip more easily through air. Today’s most efficient hybrid cars use many of these strategies, such as:

Honda Insight: Adding side skirts

What A Drag

Toyota Prius: Rounding the edges of the front end

Based on their aerodynamic design, hybrids have a head start in the race for fuel economy. The typical modern automobile has a coefficient of drag (cd) of 0.30 to 0.35—and a light truck commonly has a cd of 0.40 to 0.45. (Lower numbers mean less resistance.) The Honda Insight, Toyota Prius, and Honda Civic Hybrid have cd’s of 0.25, 0.26, and 0.28 respectively. These are the only three vehicles currently rated above 50 miles per gallon.

Conventional vehicles don’t need hybrid technology to benefit from smart aerodynamics—just a commitment to good design from automakers.

The least aerodynamic vehicles on the road are SUVs and large pickups. For example, a Hummer H2 has a drag coefficient of 0.57. Given how much front area larger vehicles cover (which mean a lot of resistance to air), small changes in design could have a dramatic effect. In fact, many analysts say it’s cheaper to decrease drag coefficient of these larger vehicles than for smaller cars. G.M. claims its 2007 Chevrolet Tahoe slips through the air eight percent more efficiently, with the result of a three percent fuel economy improvement. The new four-wheel-drive Tahoe will get 20.3 miles per gallon, compared with 18.2 miles for the 2004 model.

So if it’s so cheap and easy to modify the shape of the vehicle for reduced drag—with almost all of the cost going to upfront engineering rather than production costs—then why don’t we see more of that?

The High Price of Cool

The answer: marketing. Car designers and automobile marketing departments—in their efforts to make their products look "cool" or "different"—usually win battles with engineers trying to squeeze a little more aerodynamics and fuel efficiency out of their designs. Many people think the Volkswagen Beetle’s iconoclastic design looks cool, but the cd of 0.38 can’t touch a Prius’s 0.26. Ford engineers had to work like mad to improve the engine efficiency of the new F150 pickup truck, just to make up ground lost for F150’s aggressive looks and poor aerodynamics. If Ford put a new F150 engine in an old F150 pickup body, they would get an immediate bump in fuel economy.

The carmakers would argue they couldn’t sell cars without offering new styles. If this is true—and if gas prices continue to swing wildly up and down and up again—then the "aero" look might quickly come into fashion. And boxy blunt designs might go the way of leisure suits.

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