At speeds greater than 40 miles (64 kilometers) per hour, less fuel is used in driving an automobile with the air conditioner on and the windows up than with the windows rolled down. This is due to the air drag effect—the resistance that a vehicle encounters as it moves through a fluid medium, such as air. In automobiles, the amount of engine power required to overcome this drag force increases with the cube of the vehicle’s speed—twice the speed requires eight times the power. For example, it takes 5 horsepower for the engine to overcome the air resistance at 40 miles (64 kilometers) per hour; but at 60 miles (97 kilometers) per hour, it takes 18 horsepower; at 80 miles (128 kilometers) per hour, it takes 42 horsepower. Improved aerodynamics, in which the drag coefficient (measure of air drag effect) is reduced, significantly increases fuel efficiency. The average automobile in 1990 had a drag coefficient of about 0.3 to 0.35. In the early 1960s it was 0.5 on average, and it was 0.47 in the 1970s. The lowest maximum level possible for wheeled vehicles is 0.15.