I live in Texas and am less sensitive to heat than many folks, and I ride around with my driver’s side window open and the A/C off year-round. I don’t open the other windows because I don’t have power windows and would have to get out and walk around the car to get them all open. The reason I do this is partly for environmentalist and economic reasons, but mostly due to the fact that my little '91 Civic simply does not like to accelerate when the A/C is on.
But I am constantly being told that I’m getting worse gas mileage with the window(s) down than I would by running the A/C. The know-it-alls who tell me this are apparently assuming that I have all four windows down, which I don’t.
I recently travelled to Dallas (about a 4-hour drive from my hometown) and back. On the way up, I was in the sun and so I alternated between having the window down and having the A/C on. The A/C was on for a total of less than an hour during the entire trip. I burned 3/4 of a tank of gas. On the way back, I eschewed the A/C and kept the window down the whole way. I had 1/2 a tank left when I pulled into my driveway, having filled up just before leaving Dallas.
So what I’m wondering is, have there been studies on gas mileage of a car with only ONE window down? Clearly my mileage was greater with the A/C off, but is this just a peculiarity of my quirky little car?
I recently watched an episode of Mythbusters on TV and they addressed this question. Two cars of the same make and year, fuel tanks drained and refilled with a measure of gas. Both driven at a constant speed around a closed track. Their results suggested the car with A/C got far less mileage than the open windows car, as the A/C-less vehicle continued to lap the A/C car several laps after the fuel ran out. I realize your question was more to the “one window” discussion but it seems that even with all the windows down, the A/C-less car did better. I don’t know if that program and test are available online.
Ultimately, it would depend on such factors as the design of your car and where you’re driving. Rolling down the windows will have a greater relative drag effect on a Corvette than it would on an H2. A smaller car (I’m assuming) uses less energy to keep cool than a larger car would (smaller cabin, less air to cool down, less energy used for cooling it).
On a recent trip to Houston, I found that I burned a quarter of a tank getting there and half a tank coming back. Part of this is no doubt the fact that I got lost on the way back (but then, I also got lost, albeit to a lesser degree, going down). part of this might just be that I was driving uphill (I’m assuming that College station is relatively uphill from Houston, being farther from the coast. Could also have been traffic.
Then again, it might have been Ninjas siphoning fuel out of my gas tank while I was on the interstate.
The bottom line is that the SD column in question is a classic example of Bad Science. Quite unfortunately, not only has it not been buried, never to see the light of day, but it gets referenced all of the Net.
Why it keeps getting relisted in the front page is a total mystery to me.
As far as the general question of A/C vs windows down goes, I recall reading an article on the Taurus when it first came out. The claim was that the designers paid so much attention to aerodynamics that it actually got better mileage using A/C than with the windows down. The point was clearly that it was unusual in that regard, but I can imagine that any car with a powerful engine and aerodynamic enough design might be the same.
I tend to think that rolling down one window is worse than rolling down two on the same side. I have found that in many vehicles there is an annoying beat effect with just the driver side window down. I believe it is due to air flowing in without an easy way out. Pressure builds until it is great enough for air to flow out the open window, at which point it flows out until the pressure drops enough for air to flow in. The oscillations can even be painful. Whatever actually happens, it is hard to believe you get as smooth flow as you would by rolling down the window in the rear.
They revisited this on one of their shows that was on last night. It turns out it depended on the speed of the car they tested. IIRC, at 60, a/c with windows closed gets better mileage, as Cecil showed; and at 40, no a/c and windows open gets better mileage.
