It gets pretty hot here in N. Va (its the humidity). I think the A/C does not work "harder" no matter what the conditions are. These things do not have thermostats like your home A/C. You are the controller of the temp with the knobs. When you turn it on it just works at its rated capacity until you turn it off. When you dial in the temp knob to adjust the cabin temp, all you are doing is just letting in more outside air to dilute the cold air from the unit. The unit is still working at its same capacity, you are just diluting its output. It does not cycle on/off unless you turn it off. In the Max A/C mode, you are just recylcing inside air and boosting the fan, cooling things faster until you find it too cold and go back to the regular mode. In regular mode, if you come to a stop light it turns off and the car can warm up quickly on a hot day. In a really hot climate, it may take longer to cool the car, but it will work at the same rate and burn the same gas as in a cooler climate.

These are just my feelings after having driven A/C cars for a while, but none of them had temp controls, so I may be wrong about those cars with them. I have not noticed huge mileage difference between A/C and non-A/C mode. I sure would not want to spend much time in a non-A/C car in the 90/90 days we have here (percent humidity/temp).

Dave