At highway speeds, most of the energy needed to move a car down the road goes to pushing air out of its way. (See Energy Requirements). On the EPA highway cycle with an average speed of 48 mph, 54% of the energy required to move a car goes to aerodynamic drag. Because drag increases with the square of speed, more than twice as much energy would be required to overcome drag at 70 mph.

Three factors determine a vehicle's drag: 1) its speed, 2) the cross-sectional area it presents to the wind, and 3) its drag coefficient or Cd. The drag coefficient is a measure of the "overall slipperiness" of a vehicles shape.

Aerodynamic Vehicles of the Future

Concept vehicles developed as a part of the government/industry cooperative partnership for a new generation of vehicles have 40% less aerodynamic drag than a conventional vehicle.