Drag coefficient is a measure of a vehicle’s resistance to the air through which it passes. The sleeker shape of current cars reflects an aerodynamics movement begun 20 years ago, in part, to improve fuel efficiency. Drag, no different than a stuck parking brake or any other form of resistance, works against the engine’s effort. The higher a vehicle’s coefficient of drag, the harder its engine works and the more fuel it burns.

A vehicle’s coefficient of drag, abbreviated Cd, is influenced not just by its size and shape but by the surface of its underside and how high above the ground it sits. Ground-hugging vehicles tend to be more aerodynamic. Vehicle designers use data collected in wind tunnels to shave hundredths of a point off the Cd of future models.

One of the most aerodynamic production vehicles sold today, with a Cd of 0.26, is the Toyota Prius. Currently the most fuel-economical vehicle sold, the Prius is aerodynamically styled to cheat the wind. But not all cars have to look so fanciful to rate well. The Mercedes-Benz S-Class sedan is claimed to measure an impressive 0.27 — due in part to a suspension system that lowers the car’s body at cruising speed. The Chrysler PT Cruiser rates 0.39, and sport utility vehicles tend to climb well above 0.4.

Information for this was taken from Cars.com’s glossary, written by Joe Wiesenfelder.