When you're driving around town at relatively low speeds, you'll use less gas by switching the air conditioner off and rolling down the windows [source: Arthur]. It's more efficient to drive with the windows down at slow speeds as opposed to faster speeds because there's less aerodynamic drag when you're driving slower [source: Motavalli].
As your speed increases, however, the amount of drag on the vehicle will also increase. But the drag doesn't increase in a linear fashion, it increases exponentially. For example, when your vehicle is traveling at a speed of 70 miles per hour (112.7 kilometers per hour), there's actually four times more force on the vehicle than when you're cruising around at 35 miles per hour (56.3 kilometers per hour). So even though the vehicle's speed is doubled, the drag is actually increased by four times.
If you're searching for a good rule-of-thumb number for when it's best to open the windows and switch off the air conditioner, according to some experts, the cut-off should be around 40 miles per hour (64.4 kilometers per hour) [source: Arthur]. What's the reason? Well, at low speeds your engine is producing less power, so it would have to work much harder to power accessories like the air compressor. When the engine is operating at faster speeds, it's already producing ample power for both the engine and additional equipment [source: Austin].
Although we've made the case for both windows down and air conditioning, some argue that the windows down option is still the better bet. Car and Driver did its own study and determined that you should switch off your air conditioner -- most of the time [source: Austin]. However, the U.S. Department of Energy recommends using both windows down and air conditioning when the conditions permit [source:U.S. Department of Energy].
So, if you're conscious of going green to reduce fuel consumption, or if you're just wanting to save some green and make fewer stops at the pump, then combine both windows down and air conditioning use. If you're smart about when you choose either option, you'll save a little bit of gas and stay cool while doing it, too.
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