Your A/C compressor causes parasitic drag on your engine. It is turned (thus compressing freon and allowing cooling) via a belt that is attached to your crankshaft. How much effect this will have on your gas mileage will depend on how hard it is to turn your compressor, and how much power your engine is creating. I personally DO experience a decline in my gas mileage with my A/C on, but I have an old car with an old compressor and an engine that doesn’t have a lot of power to spare. If you drive a Diesel V8 truck with a small cab (and a small A/C compressor to match), you will probably not experience a difference. Wild claims meant to apply to all cars on the road under all conditions are silly. That being said, if you want to make a blanket statement, an accurate one would be that running your air conditioning will result in a net loss of power from your engine, which may or may not manifest itself in your gas mileage.

The “windows down” debate is very similar. Aerodynamics barely even matter when you are traveling at slow speed because air is extremely fluid. However, the air applies 4 times more resistance for each doubling of your speed. So, when you are going 80 on the freeway, there’s a good chance that the utter destruction of your modern car’s very good aerodynamics will matter. However, at a constant speed, the parasitic drag of your A/C compressor isn’t likely to cause a lot of drag because your engine is not being used to accelerate the car (which requires a great deal more power than maintaining your current velocity).

If you absolutely can’t handle using your brain to figure out how this applies to you, consider this rule of thumb: If you are going to be putting around town, it’s probably better to just have your windows down. If you’re on a cross-country road trip, turn on the AC… the 1 or 2 mpg you might lose isn’t likely to kill you.

http://www.omninerd.com/2006/07/16/articles/57

i find it a lot quieter when traveling at highway speeds with the windows closed and the ac on.

the idea is that if you’re not using AC, you have your windows down, which causes drag. they determined that all in all, windows down gives you an extra MPG or two over AC use.

while the mythbusters tests aren’t always extremely scientifically sound tests, i have a feeling that windows down and AC on weren’t compared in the CR/edmunds tests, so neither set are ideal or thorough. so take both with a grain of salt, imo.

I’m pretty sure #1 is wrong. When I run my AC I get 1-2 mpg less than if I run nothing at all. Now, my car only gets about 20mpg at best anyway, but you get the point. I know I’m only one “rather subjective” data point, but that’s all I’ve got.

#2 also seems off. The more aerodynamic your car is, the more open windows will have an effect on your mpg.

I guess in fairness, it does say “little effect”, and if little is somewhere