Gidjit's AC compressor doesn't engage unless I push the AC button to "ON". Otherwise I just get cool fresh air from the outside. I've run the AC through the summer and experienced 36MPG on average. When the weather cooled to below 70 then the AC was turned off and my MPG jumped to 39-40. It's been raining alot lately so that requires me to run air through the defrost to keep my windscreen clear. I'm still getting 39-40 MPG.