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On the freeway, is it more efficient to drive with the windows up & A/C on, or the windows down & A/C off?

I've heard both sides of this issue, but never from a really educated source... Any mechanics, please help. I drive a '98 Chevy Lumina 3.1 V6... Thanks.

Public Comments

1. It is more efficient to have the windows up....this creates less drag.
2. I have always been told that at high speeds it is better to use the a/c. The weight of the wind through the windows adds extra drag to your car and makes the car work harder. I am not a professional, but that is what I was told by my mechanic.
3. mythbusters did two shows on this issue & i believe that they determined that under 55mph, it's better to have ac off & windows down, but over that speed, the wind resistance makes it more efficient to have the ac on and winds up. sadly through work, i can't access the mythbusters site to verify the speed, but you may want to browse their page to make sure i'm right that it's 55mph.
4. There are two answers. At low speeds, windows open. At high speeds (40+) A/C on and windows closed. Wind drag increases exponentially as vehicle speed increases. Where A/C use draws a constant amount of power. At some point, the wind drag exceeds the amount of power the A/C uses...
5. on the freeway, windows up, a/c on. in town, windows down a/c off. if it's blistering hot, doesn't matter, a/c on.
6. I saw the same episode of mythbusters and don't remember speed being a factor. So it seems always better gas milage with the windows down. I was told by an auto motive instructor, ASE certified no less, the oppisite.