What determines the duty cycle of the A/C compressor?

[LtNoogie]

Originally Posted by BailOut

Actually, running the a/c is a 6-11% hit on MPG any time the compressor is cycling, which is much more often than not. That makes it exponentially less efficient than your initial thinking, and puts it well into the realm of measurably using more energy under "normal" operation, and measurably wasting energy if one has the windows down at the same time.

But the question boils down to whether or not windows down affects how often the compressor is running.

If I run the A/C on fresh, the drop in temperature across the evaporator is purely based on the input of outside air.

If I run the A/C on recirc, I can see how windows down would not allow air conditioned air from being the cooler input to the evaporators.