OK, this question has bothered me for years.

Which is more effecient for MPG in the summer:

drive w/ your windows down?
drive w/ your AC on?

I know the AC takes power away from the car, but the w/ the windows down, it drags. Which one do you think, any ideas?

 

In the city you shouldn't have any problems running with the windows down, but at higher speeds on the freeway run the A/C. Supposedly it gets better gas mileage - maybe 1mpg. It was in the manual for my old Z. =)

 

I never roll the windows down anymore. I have even heard that the AC can help improve mileage on newer cars. I don't think new compressors really affect the motor much anymore. I always use the AC and get great mileage out of all my cars. But of course I turn it off when I depress the accelerator at a rapid rate! Haha. I mean hit the loud pedal!

 

How much does the AC affect a car's performance?

 

I don't know about brand new cars since everything is computer controlled these days. I would subtract a good 5 HP on older cars, say, pre-mid 90's. It just creates that good old parasitic drag on the motor which can slow revs and make the car slower. I would not think it would be a lot, but when you race, every .5 HP counts!