At highway speeds, your vehicle is fighting an invisible foe trying to hold it back. The nemesis is wind resistance, and it is an automobile's worst enemy. Overcoming the resistance of the air is the key to increasing acceleration, top speed, and improving fuel economy.

Wind resistance is often measured with sophisticated computers in multi-million dollar wind tunnels. The result is usually expressed as a number called the "drag coefficient" (Cd). While having a low drag coefficient is important, the size of the vehicle (expressed as "frontal area") is also important. In general, as drag coefficient and frontal areas decrease, a vehicle becomes easier to push through the wind. Automakers are well aware of this, so they go to great lengths to ensure a low Cd on vehicles designed for high efficiency. Some approximate Cd values for well-known vehicles.

• .25 Honda Insight
• .26 Toyota Prius
• .27 Nissan GT-R
• .34 GMC Acadia
• .36 Bugatti Veyron (spoiler retracted)
• .36 Chevrolet Suburban
• .48 Classic VW Beetle
• .65 Tractor Trailer (with fairings)
• .75 Formula 1 Car (downforce = high drag)

There are a couple of ways to determine the Cd of your vehicle. If your vehicle is stock, simply look it up on the Internet. If you have modified your car with wider tires, spoilers, roof rack, or changed the ride height, then your Cd has changed. To determine whether your mods are hurting or helping you at the pump (or the track), why not calculate it yourself with a few household items and an Excel spreadsheet? Of course, this requires a bit more effort... but, if you have any geek in your bloodstream, this will really get your juices flowing. The process is too long to detail here, but check out the simple instructions, grab your household items, and let us know what your results are. Thanks for the tip, Farris!

[Source: Instructables]

How To: Measure your vehicle's Cd without a million-dollar wind tunnel originally appeared on Autoblog on Tue, 22 Apr 2008 11:01:00 EST. Please see our terms for use of feeds.