I know that if you drive over 50mph it is suppose to be more efficient to have the windows closed and the AC on rather than the windows open and the AC off. But my question is, does it matter how high I have the AC on or does it use the same gas when set to low or high?

 

You mean high or low temperature or fan setting?

The lower the temp in auto the more the compressor has to run. But unless the car is low powered or a 4 banger, most compressors today are so efficient that it hardly matters when or how much it runs.

 

It depends as bit says, on the engine. While the 3.3L in my RX seems to ignore the AC, my old 5-spd '95 Civic EX's 1.5L would cruise at 70-75 all day on the highway at about 35-36 mpg in winter, but in summer's heat that dropped to 32-33 mpg with the AC compressor thumping away. It helps when the power consumption of your compressor is not such a significant part of the engine load.

As far as opening a window, that's fine at urban speeds, but the buffeting you get from an open window can be tiring. Experiment with window/moonroof openings to keep this effect to a minimum. My old Suburbans had power tailgate windows that could be opened about two inches - and when combined with two open front windows you could trim that tailgate opening to achieve perfectly still air in the cabin.

Well, you had to do something for excitement . . .

 

makes the most difference on a lower hp/torque engine, I doubt the larger higher torque engines would experience any mpg loss