There are two main factors to consider when approaching this question. The first deals with how the air compressor in your car works and how much extra fuel the engine has to use to keep it running. The second is what is known as air resistance or drag. Drag is the resistance that cars, and all moving objects, encounter when moving through the air at any speed. Most modern cars are designed to be relatively aerodynamic, which allows them to pass through the air with minimal resistance.
However, when a vehicle has its windows down, air passes into the car where it was formerly allowed to flow over it, causing resistance that didn't exist when the windows were up. You can think of it a bit like a parachute. When a skydiver opens up the parachute, it cups the air and causes a massive amount of drag, enough to slow the speed of the skydiver and allow him or her to land safely on the ground. Unlike the parachute, you definitely don't want a lot of drag on your car because it makes your engine work harder to get your vehicle up to the same speed.
So, does drag really affect a car's fuel economy more than air conditioning? There are times when using the AC will save you gas.
Back in 2004, the Society of Automotive Engineers (SAE) conducted a study at a General Motors wind tunnel and on a desert track. In the wind tunnel, air was forced over the front of the car and also from an angle on the front of the car to simulate a cross wind. In the desert, temperatures and vehicle speed were factored into the study. Two vehicles were used in the test, one was a full-size SUV with an 8.1-liter V-8 engine and the other was a full-size sedan equipped with a 4.6-liter V-8 engine. Overall, both studies showed that driving with the windows down has a significant negative effect on the fuel efficiency — more than using the vehicle's air conditioner.
For the sedan, when the windows were down, the efficiency was reduced by 20 percent, while the SUV fuel efficiency was reduced just 8 percent [source: Hill]. These differences are an important factor in determining just how much the windows down option will affect the fuel efficiency of your vehicle. The study concluded that the more aerodynamic the vehicle, the more drag open windows will create.
However, with newer, more efficient vehicles the difference is likely to be negligible.
Since no tests have been conducted on an extensive array of vehicles, here are some :
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. 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].
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.
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