From the August 2008 issue of Investment Advisor • Subscribe!

Driving Green on Regular Gas

We can't all be driving electric, hybrid, or biofuel-powered cars, yet, but there are things we can do to reduce the amount of energy that our conventional vehicles use, starting with driving slower. I know it's hard, but you can get used to driving at 50 or 55 mph, and taking an extra few minutes to get somewhere won't kill you, either.

One point of automobile contention is the open-window-versus-air-conditioning debate and the effect of each on fuel consumption. It's one of those things that everyone has an opinion on, but not everyone has any facts to back them up.

According to Brendan Koerner, writing for "The Green Lantern" on slate.com, the best rule of thumb is to breathe in the fresh air while in local traffic and switch on the A/C when you take to the highway.

"Depending on your vehicle's design, an active A/C can cut fuel economy by anywhere from 3% to 10% percent in standard summertime temperatures. During a brutal heat wave, though, the power drain can be near 20%--the hotter it is outside, the harder the A/C needs to work at maintaining your cabin climate," he writes.

The reason you don't want the windows open on the highway is that as the car accelerates, drag (which increases with the square of the speed) becomes more of a factor and the car has to work harder to battle wind resistance.

The rest of Koerner's column illustrates how hard it is to get credible information on green driving. The test data he was able to find, from the Society of Automotive Engineers, dealt with only two vehicles, an SUV and a full-size sedan, both with eight-cylinder engines, so it's a little hard to determine exactly how that translates to the dozens of other auto types that Americans drive. At least we have a rule of thumb.

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