A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

What gets the better gas mileage: windows up, AC on, or windows down, AC off?

June 13, 1986

Dear Cecil:

No doubt this query won't have the same mass appeal as some of your other columns, i.e., sneezing after orgasm, but I feel it merits a moment of your time. On a recent excursion across the highways of this state, my traveling companion posed a perplexing problem. She contends that the average automobile is much more efficient with the windows down and the air conditioner off. I, on the other hand, maintain that the aerodynamic drag created by having the windows down makes the savings marginal at best. Help us, Cecil, as our discussions are getting rather … heated.

Cecil replies:

You'd think it would be pretty easy to get to the bottom of a question like this. Just call up Detroit and ask, right? Guess again. All you get for your trouble is loads of contradictory doubletalk.

But never fear. Cecil recently arranged (at no little expense, I might note) a cross-country expedition for the express purpose of determining which gives you the bette gas mileage: having the air conditioner off and the windows down, or the AC on and the windows up. For purposes of comparison, we also checked the mileage with the AC off and the windows up, which theoretically would give you the best mileage of all. In between these tests, we spent some time contemplating the Atlantic in lovely Rehoboth Beach, Delaware.

The car was a four-door Pontiac 6000 LE with 1,600 miles on it at the start. We drove 300 miles in each of the combinations described above, maintaining a 60 MPH speed on more or less level interstates throughout. Here's what happened:

Windows up, AC off: 35 MPG
Windows up, AC on: 34.4 MPG
Windows down (all four), AC off: 33.8 MPG

These figures are preliminary, of course, but interesting all the same. We got slightly better mileage running the AC with the windows up than we did with the fresh-air method — a surprising result, some may feel, but there you have it. Still, the difference was slight, and in my humble opinion, statistically insignificant. I should point out that it wasn't very hot outside, and we didn't have the air conditioning on at full blast. If we'd run it at arctic maximum, I suspect the mileages would have turned out to be identical.

Unfortunately for you, M., I ran out of vacation before I was able to prove this idea conclusively. Pending further investigation, the best I can do is declare your argument a draw.

The argument heats up

Dear Cecil:

As researchers at the Florida Solar Energy Center, we have conducted an experiment that sheds some light on whether running the car air conditioner during freeway driving has less effect on fuel economy than turning the AC off and rolling down the windows, which increases drag. Our answer, which was obtained using a VW GTI under repeated testing with fairly accurate instrumentation, is contrary to yours. Although rolling down the windows reduced mileage, the reduction at 67 MPH (3%) was not as great as that caused by running the AC (12%). The answer could vary depending on the automobile. One would assume that a different answer might be obtained using a very aerodynamic car with a large power plant (engine load from AC is a smaller fraction of overall power output).

Cecil replies:

As scientists, Dan, we accept calmly the possibility that our results may not be replicated by other researchers. So you won't hear us saying you guys are lying scumbags. We note that our car had a bigger engine than yours, and that we were doing our testing in Ohio in May, not (as in your case) south Florida in July. Obviously more research is called for. Unfortunately, by the time we get around to it, we'll probably all by using air cars powered by nuclear reactors.

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