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Fuel Efficient and Economical: Windows Down or A/C On?

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Which option is more economical? Window up and the air conditioning on, or windows down and no air conditioner?

The first has to deal 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. Phenomenon is just like parachute.

Window Down A/C On

Based on a study conducted by the Society of Automotive Engineers (SAE), driving with the windows up and the air conditioning on is typically a more fuel-efficient way to drive. The SAE study was conducted 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. Overall 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. 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.

When driving at speeds of more than 70 kilometers per hour with the windows down, there’s a decrease in fuel efficiency of 20 percent or more. Although using the air conditioner decreases fuel efficiency as well, cooling the air through the compressor only decreases the fuel efficiency by about 10 percent. So, when traveling at
speeds around 70 kilometers per hour or faster, air conditioning is usually a better bet.

When you’re driving around town at relatively low speeds, you’ll use less fuel 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.

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 100 kilometers per hour, there’s actually four times more force on the vehicle than when you’re cruising around at 50 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 60 kilometers per hour. Because 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.

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