Thanks to Green Lantern for debunking the "A/C is better than open windows" myth for low speeds. But I and other engineers here in Albuquerque who care about mileage continue to think the myth is harmful at highway speeds as well.

With so many cars carrying decent fuel economy instrumentation these days, you would think more people would notice what we have seen: the air conditioning compressor causes an immediate and substantial increase in fuel consumption every time it turns on, while moderate window opening causes so little extra drag we cannot confidently even detect it even highway speed.

So who am I, a mere retired Electrical Engineer, to disagree with professionals? Notice that the more carefully documented claims for bad effects of windows appear to test the condition of all windows all the way down. But I've settled on two windows down an inch or two, both on the same side, as my preferred travel configuration. In both of our cars (2002 Audi A4, 2006 Prius) that seems to set up a circular circulation in the cabin, giving plenty of fresh air to all seats. Thought I have not proven it by test, I believe the added air drag to be much less than with all windows down. Also wind noise and buffet is dramatically less at highway speed.

Aside from living in the dry southwest, the key to making part windows acceptable as an A/C alternative is parking in the shade if at all possible. When I can do that, I think part window travel comfortable almost every warm day of the year here in Albuquerque. When it is comfortable, I am very sure it is cheaper, even at 80 mph in these vehicles.

Yes I know about the drag square law (got taught in freshman Aero & Astro class at MIT). And yes, I realize that means there is a crossover speed at which A/C is better. I just think it is far higher, especiallly at part window, than most people imagine. I think for part window, it may be over 100 mph on my Audi, and not attainable on my Prius.

Living in the hot humid Southeast, I'll stick with the A/C, and spend a little more on gas.

Not sure where everyone keeps getting this high speed amount for the difference,but I remember seeing on a Mythbusters episode a while back that an SUVs like my explorer the tipping point is typically 55 mph so that isnt to bad for me.

A couple observations--

You live in Albuquerque. So your air conditioner is mostly just cooling the air, not dehumidifying it.

What color is your Audi? I have begun to notice a difference in cracking the windows between my navy Escort and my husband's gold Malibu.

A light colored car in a dry enviroment is likely going to feel more comfortable than a dark colored car in a muggy enviroment, all other things being equal.

(Note: the navy escort doesn't have working AC, so I've very aware of the different combos of window height, speed, fan setting, and enviroment in keeping myself cool.)

ASlyJD:

What color is your Audi?

light grey

And, yes, I know that A/C dehumidifies air and use that for windshield defogging at need.

My point is not mostly to press people to roll down windows instead of using A/C. My target is people who think they are saving gasoline by using A/C as an act of economy or environmental responsibility. If they could change their behavior and be comfortable, I wish they realized that they woudl also be saving fuel. No hair shirt environmentalism for me--I'm a conservative Republican.

You can drive around in Albuquerque with the windows just rolled down a couple of inches? You are a better man than most...

How do you deal with whole "green house" effect? If it is over 100 degrees, the inside temp, due to the glass will be significantly higher. And, as you are driving around with your windows rolled down a couple of inches, aren't you just recirculating 100+ degree air? Where does the "cooling" effect come in? [A couple of inches doesn't provide a significant amount of air movement for convection/conduction cooling.]

What do you do in traffic? When you have to stop for periods of time?

I don't know many people who can stand prolonged exposure to an automobile in outside temps above 100 degrees and windows only rolled down a couple of inches.

Doc Holliday:

You can drive around in Albuquerque with the windows just rolled down a couple of inches? You are a better man than most... How do you deal with whole "green house" effect? If it is over 100 degrees, the inside temp, due to the glass will be significantly higher.

It is quite rarely over 100 degress F in Albuquerque. This is not Phoenix, depending on the place in town we are 4000 to 5000 feet higher, and in high summer that gives almost 15 degrees lower temperatures.

A couple of inches doesn't provide a significant amount of air movement for convection/conduction cooling

Ah, but it does. Try it!.

What do you do in traffic? When you have to stop for periods of time?

Stoplights may take about a minute and a half. We think our free way is severely concested if an eight mile trip takes an extra five minutes because much of it was at 45 and a few times it dipped to 15 or a momentary stop.

To re-iterate, I'm not advocating intentional suffering, just suggesting that people freely give windows a try when it works for them, not falsely thinking they are wasting gasoline.