What is cD and why does it matter?

Aerodynamics. When discussing overall efficiency, sometimes it's easy to overlook just how important the shape of an automobile is in determining how fuel efficient it is

First, let's discuss what the term aerodynamics means. According to Merriam-Webster, aerodynamics is "a branch of dynamics that deals with the motion of air and other gaseous fluids and with the forces acting on bodies in motion relative to such fluids." Clear as mud? In this case, the fluid we're talking about is indeed air, and the easier it is for an object to cut through the air, the less energy is required to keep that body in motion.

For the purpose of comparing different automotive designs, engineers measure the resistance of a body to flow through the air using computational fluid dynamics simulations and wind tunnel testing. A flat board held perpendicular to the air flow may have a coefficient of 1 or more, depending on the shape of the edges and the surface texture. The drag coefficient is a unitless number and is based on the shape and surface properties of the object. Two identical objects that are different sizes will have the same cD. However, that doesn't mean they have the same overall drag. 

This, of course, is further complicated by the vagaries of the real world. The addition of items like outside mirrors, windshield wipers and radio antennas can cause a lot of disruption to air-flow. There is also the fact that cars often don't travel perpendicular to air-flow. Vehicles often encounter cross-winds when driving down the road so the airflow must be measured at various angles to ensure resistance is kept to a minimum. 

Safety is also a concern.The air-flow over a vehicle's body can trigger either lift or down-force just as it does with an aircraft wing. If a vehicle's shape causes too much lift, it can make the vehicle unstable and difficult to control. All of these factors must be balanced in the final design.

The designers of all of the most fuel efficient vehicles ever offered for sale have taken aerodynamics very seriously and as such, they all have a very favorable cD. For instance, the GM EV1 scored a cD of 0.195, which is quite good. For comparison, the 2010 Toyota Prius manages a fine 0.25 and the Aptera 2e (above) blows them both out of the water air with an amazingly low 0.15. When multiplied by the car's frontal area, the Aptera design scores even better due to its narrow, bullet-like shape with narrow out-rigger front wheels that are completely shrouded . For what it's worth, the brick-shaped HUMMER H2 scores a dismal 0.57 – further proof that its designers were in no way concerned with its fuel efficiency.

Besides the actual bodywork of an automobile, there are other factors contributing to the overall coefficient of drag any given car is able to register, including the car's tires and its ride height. Wide tires move more air and therefore take more power to move, and the air pressure of a given tire can have drastic consequences on its ability to roll. 

Why is all of this so important? Calculations reveal that about 60 percent of the energy used to move the average car goes towards overcoming its aerodynamic drag. That's huge, and means that even small improvements in a car's cD can pay big dividends in overall fuel efficiency.