Which lets me get better gas mileage, having the windows down or using the a/c?

I think it really depends. At low speeds, windows down is better. The faster you go the more it changes. The biggest problem at high speeds is wind resistance. Having your windows open decreases the aerodynamic efficiency of your car, meaning you have to work harder to move through the air. The faster you go, the more of a problem this becomes. At some point you will reach a speed when it makes sense to roll up the windows and turn on the A/C as the aerodynamic improvement will outweigh the added drag from your A/C. Exactly what speed that cross over occurs will vary greatly depending on the car.
Which lets me get better gas mileage, having the windows down or using the a/c?
AC on windows up.
Which lets me get better gas mileage, having the windows down or using the a/c?
Louis G is correct. They had this myth question on FOXNEWS the other day. A/C uses the same amount of gas as having the windows down. Of course at certain speeds. But still if where you live is hot. Just turn on the A/C the efficiency wouldn't be enough to notice.

ginkgo