The Toyota Prius' unique shape is one factor that helps it get incredible fuel economy.

­©­iStockphoto.com/TIM MCCAIG

The Coefficient of Drag

­ ­We've just learned that the coefficient of drag (Cd) is a figure that measures the force of air resistance on an object, such as a car. Now, imagine the force of air pushing against the car as it moves down the road. At 70 miles per hour (112.7 kilometers per hour), there's four times more force working against the car than at 35 miles per hour (56.3 kilometers per hour) [source: Elliott-Sink].

The aerodynamic abilities of a car are measured using the vehicle's coefficient of drag. Essentially, the lower the Cd, the more aerodynamic a car is, and the easier it can move through the wall of air pushing against it.

Let's look at a few Cd numbers. Remember the boxy old Volvo cars of the 1970s and '80s? An old Volvo 960 sedan achieves a Cd of .36. The newer Volvos are much more sleek and curvy, and an S80 sedan achieves a Cd of .28 [source: Elliott-Sink]. This proves something that you may have been able to guess already -- smoother, more streamlined shapes are more aerodynamic than boxy ones. Why is that exactly?

Let's look at the most aerodynamic thing in nature -- a teardrop. The teardrop is smooth and round on all sides and tapers off at the top. Air flows around it smoothly as it falls to the ground. It's the same with cars -- smooth, rounded surfaces allow the air to flow in a stream over the vehicle, reducing the "push" of air against the body.

Today, most cars achieve a Cd of about .30. SUVs, which tend to be more boxy than cars because they're larger, accommodate more people, and often need bigger grilles to help cool the engine down, have a Cd of anywhere from .30 to .40 or more. Pickup trucks -- a purposefully boxy design -- typically get around .40 [source: Siuru].

Many have questioned the "unique" looks of the Toyota Prius hybrid, but it has an extremely aerodynamic shape for a good reason. Among other efficient characteristics, its Cd of .26 helps it achieve very high mileage. In fact, reducing the Cd of a car by just 0.01 can result in a 0.2 miles per gallon (.09 kilometers per liter) increase in fuel economy [source: Siuru].

On the next page, we'll examine the history of aerodynamic design.