Is it more efficient, gas-wise, to drive with the windows open (causing drag) or with the windows up and the AC on (uses gas and wears the AC system)?
It depends on the car; on newer cars the A/C is very effient, and you lose 2 mpg maxiumum. Overall, I would say there is little difference. You have A/C to keep cool and comfortable, as well as have some peace and quiet. I would just use it and drive sedately; that will save far more on gas!
AC units are more efficient and cars are more aerodynamic. It really depends on the car.
Direct experimentation in your own vehicle is the best way to answer this. As a general rul, I’d say that the newer your car is and the fast you go then the more likely it is for the AC to be more efficient than an open window.
Let me add one thing to what already has been said.
Chances are the difference is going to be very small. Don’t sweat the little stuff and this is one of the little ones.
It’s really six of one, half dozen of the other. I’ll go with AC every time. Driver comfort is a saftey issue.
Mythbusters did a show on this. It is more efficient to use your AC than to ride with the drag, that having your windows down, cause on your car.
I will add that, if you are driving at very low speeds (i.e.–no more than 30 mph), it would be more economical to drive with the windows
down–as long as you can tolerate the heat and humidity. Once you are moving at speeds higher than 30 mph, having the windows open will really interfere with the aerodynamics of a modern car and will result in lower mpg.
That being said, the differences in mpg are truly miniscule in either scenario, providing that you are already driving in a manner that is economical. There is certainly something to be said for comfort, and when all is said and done, the added comfort is probably worth the truly minimal “penalty” for using the a/c at any speed.