The way your vehicle moves through the air has a surprisingly large effect on its fuel efficiency. That’s because any air that fails to move off the vehicle’s surface must be pushed away, and that takes energy.
Aerodynamics is the study of air moving around a solid object traveling through the air. Drag and lift are two of the most studied aerodynamic forces on airplanes, cars and trucks. Aircraft designers are concerned with balancing lift and drag as it pertains to performance. Drag is the more important force when it comes to vehicles unless we are referring to a very high performance car, for example a Formula One racecar, where making the car “stick” to the road creates the grip that allows the car to negotiate turns at high speed. Great effort is put into preventing lift on racecars using components such as air dams, spoilers and wings.
Drag opposes forward motion. The energy used to overcome drag on a vehicle places more load on the engine than any other factor in moving the car forward. Therefore, drag has the largest impact on fuel usage and mileage. Also, it takes more power at higher speeds to overcome drag. When measuring how effective a vehicle is at defeating the force of drag, engineers use a figure called the drag coefficient or Cd. It is calculated by measuring the effects of frontal area, speed and air density. Nothing can be done about air density, so the engineers work on minimizing the other sources of drag.
Thus, Cd indicates how easily a vehicle slides through the air. A brick has a Cd of 1, about the same as a box van or boxy motorhome. In the early 1930s, car manufacturers discovered the art of streamlining, although initially, they really weren’t certain what went into making the most “slippery” shape. Since then, automotive engineers have made great progress in understanding aerodynamics and reducing a vehicle’s Cd as much as is practical. Many of todays cars have a Cd of about 0.30. the larger fronts for pickups and SUV's means a typical Cd around 0.40.
One of the most problematic aspects of drag is that it increases exponentially. Driving at 70 mph causes nearly four times as much drag as driving at 35 mph.
You can't change the styling of your vehicle to improve drag, but there are some things you can do to cut the effects. Keep windows rolled up, add a tonneau cover to cover truck bed, remove bug shields front the front of the vehicle and use a streamlined roof carrier when you need to transport something on the top of the vehicle. Sometimes you can actually hear drag! Wind noise is most likely a symptom of excessive drag. You can improve your mileage by tracking down the source and fixing it.
Air is a drag! Aerodynamic airflow is a big factor in fuel consumption. If you are buying a new car, ask about each model’s Cd. The car with the lowest Cd will be quieter and more fuel efficient.
