Aerodynamics - Why All Cars Look the Same
There are two basic aerodynamic forces: lift and drag. In an airplane, both are important, but in a car, drag is usually the focus. Lift is the measure of how much upward thrust is created by the aerodynamics of the vehicle in question. Obviously, aircraft use this to fly. Drag is the amount of resistance met by the air around the vehicle as it moves - the more drag, the harder the vehicle must work to move at a given speed.
Drag is measured with a formula that produces what is called a coefficient of drag (Cd). This number is 1 or less and in automotive is often called the Cd or just "drag." A Cd of 1.0 is called the "brick" or "Model T" number and is the drag coefficient of a square or rectangular object, such as the original Ford Model T. Most of today's small and mid-sized cars have a Cd of about 0.30.
Cd is calculated using three variables: frontal area, air density, and velocity squared (actual calculations are more complex, but these are the base components). Automotive engineers work to reduce the frontal area and tune the velocity in order to achieve the best drag possible.
Why Aerodynamics Matter
The aerodynamic force on a car is the single largest factor determining the vehicle's fuel efficiency. From the "Vehicle Power Usage" graph below, you can see that drag outweighs both rolling resistance (tires) and driveline losses (including engine drag from accessories) combined.
Reducing Cd by just 0.01 results in 0.2mpg improvement. So improving the Cd of a car from 0.3 to 0.25 means about 1mpg in better economy. The most aerodynamically-efficient production vehicles today are the Toyota Prius and the Mercedes-Benz S-Class with a 0.26.
How Speed Factors In
Speed is another important factor for aerodynamic drag improvement. Speed's effect on drag is a measure of velocity squared, so the gains are huge. Driving at 70mph has four times the total drag as driving at 35mph, though most drag measures really don't affect mileage if driving under the 20-30mph range. Advocates of the 55mph speed limit cite the fact that decreasing speed from 65 to 55mph results in a 10-15% fuel consumption decrease in the average vehicle, for instance.
Most vehicles today are tuned for peak Cd at 55-62mph, depending on the car and whether miles or kilometers per hour are used as the base measurement. The Environmental Protection Agency (EPA) tests highway miles per gallon at 55mph, so most cars aim for that number in the U.S.
How Can You Improve Your MPG With Cd?
Although major improvements to drag are likely outside of the reach of most vehicle owners, there are small things that any driver can do to improve aerodynamics for better fuel economy.
Driving a little slower is definitely one of those, but also watching accessories and drag-enhancers on your vehicle is also important. When driving above 30mph, keep your windows rolled up as most cars are now made to have best Cd with the windows all the way up. At higher speeds, the drag on the engine caused by your air conditioning will likely be less an impact on your MPG than will having the windows down.
Removing items not needed, such as roof racks, additional accessories (bike racks, flags, etc.), and so forth to further streamline the car will also go a long way towards improving drag. Pickup trucks should leave the tailgate up and consider a streamlined shell or bed cover to improve aerodynamics.
The average vehicle owner, if they take steps to lower their vehicle's Cd with some of the above ideas, can increase their fuel economy by almost 1 mpg.
Aerodynamics are extremely important to the efficiency of a vehicle. This is why most cars today have the same basic shape, as the "half moon cut-off" vehicle shape has the best overall aerodynamic shape without reducing interior space or vehicle dynamics.
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