Do you use more gas with the car air conditioner on or the windows open?

Question

Somebody told me you use more gas with the car windows open than the air condition, because of the drag with the windows open.

Answer 1

air conditioner


maybe if your going like 150 mph thats true

Answer 2

Ok, I'm not a certified mechanic, an aeronautical engineer, or a physicist. but my personal experience is that it doesn't make a difference between using the AC or having the windows down. For those who believe having your windows down creates enough drag to tax your engine and use more gas, I'd like for you to prove to me that it takes less time for a car to go from 60 to 0 if the windows are down than when they are up. Most of the air resistance comes from the hood and front of the cab, not the side slipstream. Open windows are not a drag parachute. Now, AC might use more gas during heavy traffic since it does cause a drain on available power and the engine must work harder to provide the same amount of power during acceleration, but I don't see an appreciable difference during normal driving since the power gain from basic momentum would balance out the power drain of the AC. That's just my layman's opinion.

Answer 3

Dear Tom and Ray:



I appeal to your expertise to settle a discussion a friend and I had recently. He says running the air conditioner in a vehicle uses as much as two gallons of additional fuel per mile. He prefers to travel with the windows down. I concede that using an air conditioner may reduce gas mileage, but I think his estimate of fuel use is high. I also think that when you run a vehicle at highway speeds with the windows open, you change the aerodynamics and increase wind drag, which also lowers fuel economy. Since we live in a hot weather climate, and summer is fast approaching, your answer will determine whether we have a year round friendship, or just a cool weather one, as I like to arrive at my destination cool and neat rather than hot and windblown. Lois

Tom: We're siding with you, Lois. Studies have actually been done on this very issue, and the results show that driving a modern car with the windows down uses MORE gas than driving with the windows up and the air conditioner on. RAY And your friend IS way off on his estimate of fuel use. Sure, it does take some power to run an air conditioner, and that power does come from the engine (and therefore uses some additional gasoline), but it's more like one or two miles per gallon....not two gallons per mile!

Tom: And apparently, it takes even a little more fuel to overcome the increase in wind resistance that comes from driving with the windows wide open.

Ray: Then, as you well know, Lois, when you factor in the increased hair-styling and dry cleaning bills caused by that open air driving, air conditioning begins to look better and better.

Answer 4

That somebody is correct, in regards to speeds above 55 mph, with windows open there is much more wind resistance, therefore the car uses more gas as it pushes against the added resistance, however when driving throughout city traffic the AC will cause the engine to heat up faster thus work harder to keep cool, resulting in greater fuel consumption.

Answer 5

You use more gas with the A/C on because the fan is a gas engine - go figure.

Your car is less stream-lined with the windows open, yes, but the drag is not nearly as much as the A/C.

Answer 6

You are correct. Leaving the windows open causes more fuel usage.
The air conditioner is used by your vehicle's electrical system. Your alternator generates the necessary power for the air conditioning system. The alternator works and rotates at the same rate no matter if your car is accelerated or idle.

Answer 7

As far as wind resistance, the windows up would be better because the wind shear naturally goes over the top of the car. If the windows are down, the wind shear on the sides is interrupted. This will add to the resistance, and make the car work harder to maintain speed.

Answer 8

According to Mythbusters

Windows down vs. air conditioning
"Urban puzzle": it is more efficient, on a hot day, to run with the A/C on and windows up than to run with windows down (b/c of increasing car's drag).

Computer-based mpg measurements:

11.7/11.8 with A/C on and windows up
11.7/11.8 with A/C off and windows up
11.3 with A/C off and windows down
So, according to the computer, it's better to use A/C with windows up.

This was too quick and easy for TV, so they decided to stage a seven hour marathon, race-til-you're-empty duel, with Jamie driving an SUV with A/C on and Adam driving an SUV with windows down. Though, once the safety inspector intervened, it was no longer a seven-hour marathon, it was a bit slower (45mph instead of 55mph), and a lot shorter (only 5 gallons each).

Jamie's A/C car ran out of gas first -- Adam's windows down SUV ran for another 30 laps -- completely contradicting the computer mpg estimate. Computer estimate based on air flow into the engine, so it would appear that it is unable to properly model the difference between A/C and windows down.

Mythbusted

Answer 9

Depends: if you're on local city streets, it's better to have the windows open, but on highway driving, the AC is more efficient because it cuts down the wind drag.

Answer 10

My mom saw on Oprah that its true. However, I would do a test yourself and see how much gas is used either way. I also read to have the air on when on the freeway, and the windows down when on the streets.

Answer 11

This is what I heard on the news sometime or other:

Its better to use your AC while running at higher speeds as on the highway. Roll down your windows if youre running at slower speeds as in traffic or stoplights.

Hope this helps!