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Why does air friction affect a car's gas mileage?
Category: Physics
Published: December 13, 2012
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Air friction does not significantly affect a car's gas mileage. Air turbulence does. Whereas air friction is the opposing force caused by air rubbing sideways along a surface, air turbulence is the act of stirring up the air and creating pressure differences. Cars are slowed down when they stir up the air too much. Stirring up the air creates vortices and billows that take energy away from the car. This effect is known as "wake drag", "pressure drag", or "form drag".
If friction were the dominant factor, then automakers would rub their cars down with oil to make them get better gas mileage. Instead, automakers improve gas mileage by designing their cars to be more aerodynamic, meaning that they have a shape that stirs up the air as little as possible. While air friction does have much effect on cars, it can become significant for fast airplanes that produce little air turbulence because of effective streamlining. The combination of friction and turbulence is known as air resistance, or drag. A lot of energy is also wasted because of inefficiencies in the powertrain.
The textbook Fuel Economy in Road Vehicles Powered By Spark Ignition Engines, edited by John C. Hilliard and George S. Springer, states that:
In addition to pressure forces, air flow generates tangential frictional stresses along the body surfaces. For a bluff body, skin friction drag is much less than the pressure drag... Consider a typical vehicle which is boxed shape with overall dimensions of length to width to height of 3 : 1.33 : 1... we find that the friction drag contributes about 0.02 to the overall drag coefficient. This value is about 5% of the drag coefficient of a typical road vehicle.