If you're concerned about fuel economy, when (if ever) does it make sense to use your vehicle's air conditioner?

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Windows down or A/C on -- which is more fuel-efficient?

During the summertime, there's nothing like driving with the windows down, hanging your arm out the window and catching the breeze as it pulls your arm up to the sky…or is there? What about driving with the windows up and enjoying the wonder of engineering that allows a cool air conditioning breeze to chill you to the bone as the pavement scorches just a few feet underneath you?

A lot of people take the time to do a little traveling in the summer months, and that often involves taking the family car, too. With more people driving, the demand for gasoline goes up which increases the price of oil, which then leads to higher gas prices. So, during the warmer months, it's not unusual for us to be asking, "Which option will save me more money? Window up and the air conditioning on, or windows down and no air conditioner?"

There are two main factors to consider when approaching this question. 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. 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 the 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 affects a car's fuel economy more than air conditioning? Find out on the next page.

It depends on your driving speed, but there are times when using the air conditioning system in your car can be more fuel efficient than rolling the windows down.

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When to Use Your Air Conditioner

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 [source: Hill]. We'll get around to discussing when air conditioning isn't a more efficient option, but let's first take a look at what the SAE found.

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. 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.

When driving at speeds of more than 55 miles per hour (88.5 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 [source: Arthur].

So, when traveling at speeds around 50 miles per hour (80.5 kilometers per hour) or faster, air conditioning is usually a better bet, but what about when you're simply cruising around town? Keep reading to find out how you can save a little bit of fuel on those slow-speed short trips.

There are times when it's more fuel-efficient to drive with the windows down -- and the fresh air feels pretty good, too.

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When to Roll the Windows Down

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.

For more information about fuel efficiency and other related topics, follow the links on the next page.

Lots More Information

Related HowStuffWorks ArticlesSources
  • Arthur, Dani M. "Will rolling down windows save fuel or not?" Bankrate.com. July 22, 2008. (June 5, 2009) http://www.bankrate.com/brm/news/auto/20050804a1.asp
  • Austin, Michael. "Gas Pains: Mileage Myths and Misconceptions." CarandDriver.com. December 2008. (June 5, 2009) http://www.caranddriver.com/reviews/hot_lists/high_performance/features_classic_cars/gas_pains_mileage_myths_and_misconceptions_feature/(page)/1
  • Hill, William, et al. "Affect of Windows Down on Vehicle Fuel Economy as compared to AC load." July 13, 2004. (June 5, 2009) http://www.sae.org/events/aars/presentations/2004-hill.pdf
  • Motavalli, Jim. "The Air Out There: An Endless Windows-vs.-Air-Conditioning Debate." The New York Times. July 30, 2008. (June 5, 2009) http://wheels.blogs.nytimes.com/2008/07/30/the-air-out-there-an-endless-windows-vs-air-conditioning-debate/
  • U.S. Department of Energy. "Tips for Improving Your Fuel Economy." (June 5, 2009)http://www1.eere.energy.gov/cleancities/toolbox/docs/improve_fuel_economy.doc