better mileage with the air on vs. windows down?

Factual Questions
1

Hi
I recently heard on the radio that a person can get better gas mileage with the air conditioning on and the windows rolled up verses the windows down and no air conditioning on. Is this true?

2

Cecil agrees but apparently there is contradictory information as well.

3

It seems a close call but I would think a lot depends on the car you are driving…the engine it has, the aerodynamics of the car, the power of the AC compressor, etc… A Hummer, for instance, seems more like a brick on wheels and I cannot imagine its aerodynamics are all that great…rolling dow the windows I would suspect would not adversely affect its aerodynamics in a significant way. Compare that to some super low drag coefficient car where rolling down the windows would make a marked difference in overall performance.

Add to that the engine and the air conditioner. I had a car once where the air conditioner put a noted strain on the engine. My car was a four banger, maybe 110HP thing (I forget now) but when the AC kicked in you could feel the engine drag. My current car has a 245hp engine and I never notice a performance difference when the AC kicks in. I would assume the different engines would show a different mileage result with AC on as one was low power so AC took up a greater percentage of the engine’s effort than a more powerful engine.

4

I think you will hear many IMHO’s on this one. Basically most will fall into:

At low speed open your windows

At high speed close your windows and run the a/c

The line between high and low speed is anyware between 30 and 60 mph.

and the difference is so small as not to be a real factor in driving a car.

Now if you are runnign low on gas and have to make it to the next station, perhaps only running the a/c while going downhill and not wanting to accelerate would be appropiate.

5

Mythbusters’ tests were inconclusive; I suspect that it depends on many factors particular to the car, the road, atmospheric conditions, and so forth.

6

I read an advice column once in a woman’s magazine (there is no online archive) that in town it is better to leave the windows open, on the highway it’s better to use the AC and put the windows up. So, what kanicbird said.

Though having the windows open when you’re going 30 doesn’t really cool the car off very much.