I remember reading in a book when I was in junior high school that driving with the windows down was more gas-efficient than having the air conditioner running with the windows up if you kept the car under 55MPH.
Does anyone know if this figure is still true for your typical mid-size sedan?
There’s a Straight Dope Classic column on the subject: What gets the better gas mileage: windows up, AC on, or windows down, AC off?
Ahh, cool. Thanks a bunch. 
the Mythbusters tackled that a few shows back. I don’t remember exactly what their results were, but I think that it was either a) gas mileage was negligible or was actually better with the windows up and AC on.
critter42
Heh, after what I’ve heard about their testing methods, I don’t put a lot of weight behind anything preceeded by “they did a Mythbusters on this…”
Though, from what everyone tells me, Mythbusters IS quality entertainment, so I do intend to watch it and see for myself sometime.
I believe the actual results of their test showed it was better with the windows down. But this is one of their tests where they admit their limited results don’t mean it’s conclusive. I seem to remember they had to scale back their test considerably making the results really questionable.
But yes, it is quality entertainment.
Mythbusters found that driving at 55mph, it was better to have the windows down than the a/c on which went against what the computer models predicted.
I saw (most of) that Mythbusters episode, and, not only was it less entertaining that most episodes, but the value of their results for that particular test is, uh, severely limited.
We can think about it though: As speed goes up, air resistance becomes larger, while the load from an air conditioner stays more or less constant.
So clearly, it’s more efficient to open the windows on a stopped car, as speeds get higher, at some speed it becomes more efficient to switch to AC. What speed exactly is undoubtedly different for different cars, depending on the car’s aerodynamics, and the efficiency of the air conditioner.
My WAG is that it’s somewhere around 40 mph for most cars, but I have absolutely no (zero, 0, nada, zilch, goose egg, shutout, blank, oh-fer) data to back that up.
OK, but these tests (and apparently I should use the term loosely) were performed with all four windows down, right? Usually I drive with one window down only. Not that it really matters - I don’t use the AC unless it’s hot and rainy or I’m stuck without moving for quite a while, because of personal preference, but I assume you get less drag with one window only open?
The owner’s manual for my 1998 Volvo said that it was more fuel efficient to drive with the AC on than with the windows down because of the increased drag caused by the wind resistance.
So, for a 1998 Volvo S70, that’s the answer.
I saw that Mythbusters, too, and as I recall, they had two cars (SUVs, actually): One with the four windows down, the other with the AC on and windows up, running concurrently. Unlike the test described in the linked Column, they had no control group. Before those tests, they should have run both vehicles with the windows up and AC off until they stopped, then compared results after making the change. Or, run the same experiment three times with the same vehicle.
As with most factors related to gas mileage, this one is probably a wash. Keeping your vehicle tuned up, the tires properly inflated, and driving smoothly will have a bigger effect on mileage.