Aerodynamic Design

Aerodynamic design has a lot more to do with performance than most people think. After all, speed limits aren't high enough for aerodynamics to matter, right? Wrong. There's a perfectly good reason why this yellow Ferarri 360 Modena is so sleek, even if it's only going 55.

Ferarri Modena

As vehicles move, they displace the air around them. The larger the height and width of the vehicle, the more air there is to displace. Drag is the resistance to motion through the air, so less drag can mean a faster car.

Automobiles are measured by their drag coefficient, which is defined (very academically) by www.fordvehicles.com as:

A vehicle's efficiency as an aerodynamic shape, useful for comparison with other vehicle designs. It is a mathematical factor that, when multiplied by the projected square footage area of the vehicle, gives its drag force in pounds. C d is derived by measuring the drag force and dividing it by the product of dynamic pressure and vehicle frontal area. Drag coefficient (C d ) = force/dynamic pressure x frontal area; the lower the C d number, the better the aerodynamic efficiency

For most cars, this value ranges from .30 to .35. Because SUVs are usually boxier, higher, and wider, their drag coefficients are usually between .35 and .45.

Here are the drag coefficients for a few select vehicles:

Hummer H2 2003: .57
Ford Explorer 2004: .43
Ferarri 360 Modena 2004: .33
Toyota Prius 2004: .26
Honda Insight 2004: .25

That right, the hybrids are more aerodynamic than the Ferrari. For more enlightening comparisons, read our Hybrid vs. SUV article.

A Very Real Impact

Physics aside, drag has a very significant effect on fuel efficiency. As discussed in our efficiency obstacles page, overcoming drag takes up the greatest percentage of a vehicle's kinetic energy during highway driving. Drag can cost a car 60% of the energy that makes it to the drive train. The faster the car is moving, the more gasoline is exerted simply to displace air.

The key to efficient highway diving is reducing drag. The Honda Insight, a champion of aerodynamics, employs a few techniques rarely seen on other vehicles. Most noticeably, the Insight has covers over the rear wheels.

Honda Insight Rear Wheel Covers

Because the rear wheels don't do any steering, they don't need to be exposed, no matter how much people love those shiny rims. Covering them reduces turbulence ordinarily created by the rotation of the wheels. Next, much of the underside of the car is covered by flat sheets which streamline airflow beneath the car. Finally, there is the shape of the body, which mimics a falling teardrop. When a droplet of water falls, air and gravity mold it into the most aerodynamic shape, a teardrop. For the Insight, it was Honda's engineers, not air and gravity, that made the body wider in the front and narrower in the back - just like a falling teardrop.

These advanced design features give the Insight the lowest drag coefficient of any mass produced vehicle.

For additional information on the Insight's aerodynamics, Insight Central provides a comprehensive list of features contributing to the ultra-aerodynamic design.