Since no tests have been conducted on an extensive array of vehicles, here are some general guidelines:
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].
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 (for the air conditioning). When the engine is operating at faster speeds, it’s already producing ample power for both the engine and additional equipment [source: Austin].
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 mph (113 kph), there’s actually four times more force on the vehicle than when you’re cruising around at 35 mph (56 kph). 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, the cut-off should be around 40 mph (64 kph) for larger vehicles [source: Arthur]. With smaller vehicles, like the Corolla, you could use the AC and get up to 70 to 80 mph (assuming that’s a speed limit where you are) without using extra gas.
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].
Driving with the windows up and the AC off might be the most fuel-efficient way of all, but who can do that on a hot day?
Really, the benefit or hindrance posed by driving with windows down will depend on a number of variables, like vehicle shape, engine size, and compressor efficiency. Every vehicle will behave differently, so there aren’t many hard rules. No matter which method you use, practices like keeping tires properly inflated, trimming excess cargo weight, and regularly maintaining driveline components will likely see greater benefits to fuel economy.
Originally Published: Jun 23, 2009
