I rarely use the A/C in my 3 when driving by myself. I drive with all 4 of the windows down traveling in the city at 0-55mph. My trip computer for this tank of gas has been in the mid 29mpgs. It is at 29.3 now. A few days ago, when I had a passenger in the car I used the A/C and the mpg dropped down to 28.3. This was only about 30 minutes of driving! The temps have been in the 90s all while this was done. I have noticed that higher temps result in better gas mileage. I assume it is because of faster engine warm ups. I do not know how low it would be if I used the A/C all the time like most people do, but it wouldn't be pretty. So, how much power does running the A/C typically use?