What is Drag Coefficient?
A number, written Cd, that describes how easily a vehicle's shape moves through air. The lower it is, the less air resistance for a given size.
The drag coefficient measures how a car's shape moves through the surrounding air. The average modern car comes in between 0.25 and 0.3, while boxy SUVs typically land between 0.35 and 0.45. It matters most at speed, because aerodynamic drag rises with the square of speed, so a lower number helps top speed and fuel economy. Many sports cars have surprisingly high drag coefficients, since the downforce that holds them to the road also creates drag.
Shape is only half of the story. Total air resistance depends on the drag coefficient and the car's frontal area, and multiplying the two gives the drag area, the figure that really sets how much power a car needs to hold a cruising speed. Reported drag areas range from 5.1 square feet for the 1999 Honda Insight to 26.5 square feet for the 2003 Hummer H2. A bicycle and rider fall around 6.5 to 7.5 square feet. Published numbers also vary by wind tunnel, by up to 5%, so a car rated 0.30 in one tunnel could measure anywhere from 0.285 to 0.315 in another.
Carmakers have pushed the number down with concept cars. Ford and Ghia's Probe series, built from 1979 to 1985, got the 1979 Probe I to 0.25 with skirted rear wheels and pop-up headlights, and the 1983 Probe IV to 0.152, with its radiator moved to the back and its engine tilted 70 degrees to lower the hood. The name later went on a production car, the Ford Probe.
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