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Old 05-21-2011, 08:04 PM   #1
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Window Up or Down?

I like to drive around town with my windows down. I'm talking top speed is 30 to 35 miles per hour. My question is, does keeping my windows down at such a low speed really effect the areodynamics enough to lose fuel efficiency more so than if I had my windows up and my AC on.

Logic tells me that at 30-35 mph, there isn't enough resistence to cause a large enough decline in FE as there would be if I were running the AC. But that's just my own thinking. Does anyone know the answer to this as in which is the more FE manner to drive?
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Old 05-21-2011, 09:16 PM   #2
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Re: Window Up or Down?

My windows are always down and most of my driving is city. I get 47-50mpg with the windows down. Works for me.
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Old 05-22-2011, 12:51 AM   #3
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Re: Window Up or Down?

The effect of increased drag from windows being open is certainly greater at higher speeds, but drag at any speed is significant. Aerodynamics are important no matter the speed, it just so happens that they become really important when on the highway/above 50mph or so.
My experience in all the cars I've driven has been, low speeds window open, high speeds ac on. That is, if you must to keep cool. I generally use the blower fan to keep cool on all but the warmest days. If I need the window open at all, I use as little open as I absolutely have to. If I need just a little air to help clear the foggy window I crack the drivers window. If I need a lot, I open it in increments until I am kept cool enough.
When I had a car with ac I'd always try running the fan on cold without ac before turning it on.

From an FE standpoint, I think you are absolutely right.
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Old 05-22-2011, 04:40 AM   #4
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Re: Window Up or Down?

I agree with "low speeds window open, high speeds ac on".
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Old 05-22-2011, 07:52 AM   #5
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Re: Window Up or Down?

That has been my experience as well.
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Old 05-22-2011, 09:47 AM   #6
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Re: Window Up or Down?

actually it depends...in central florida, during the months of june thru august...yup, def windows up and a/c on...AT ANY SPEED!
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Old 05-22-2011, 10:47 AM   #7
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Re: Window Up or Down?

Thanks for the feedback. This confirms my thoughts on the best method. In Arkansas, it can get dreadfully hot, so in the tough months you simply cannot drive without AC unless you want to end up soaking wet with sweat by the time you get wherever you're going. I'll roll the windows down as long as I'm driving a relatively low speed and the heat isn't sweltering.
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Old 05-22-2011, 12:06 PM   #8
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Re: Window Up or Down?

You can't drive efficiently when you're uncomfortable and urgently want to reach air-conditioned indoors. Do what you have to do.
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Old 05-22-2011, 12:38 PM   #9
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Re: Window Up or Down?

On long interstate trips, I can get by OK in my Accord by switching the climate control to recirculate, fan speed to medium and manually pushing/releasing the AC button every 1 mile marker so the AC compressor is only on 50% of the time. This system works well up to 90 degrees. Above 90 degrees it is usually 2 miles on, 1 mile off.

Since most of the air is being recirculated, the humidity is much lower when the ac is off than it would be if I was drawing in fresh air.

On many 90's era cars, moving the temp slider a little toward the hot side while the AC is on will only mix a little warm air from the heater core with the cold air- it does not make the ac compressor cycle on and off as would be the most efficient way to handle the situation.
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Old 05-22-2011, 01:03 PM   #10
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Re: Window Up or Down?

Beware, in many cars, the compressor runs when recirculate is on even if the A/C button doesn't light up (just like it also runs when defogger is on).
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