[SentraSE-R]

I recirculate the air as Old Mech suggests, but run the fan at full speed, cycling the AC compressor as Thymeclock suggests. Modern climate control fans draw nearly as much current running in the #1 position as they do running in the #4 position. They inefficiently wire in resistors to the #1, #2, and #3 fan switch positions to cut current to the fan, while drawing the same amount of power usage. Ergo, you're better off running the fan at full speed.

I switch the AC on when needed when gliding or holding speed on level ground, and turn it off as much as possible when pulsing or climbing hills. That way, I'm in control of when the AC compressor is working. The other recommendations let the car's computer control the AC compressor duty cycle.

But I have a dumb climate control system. If you have a luxury car, you're hosed, because it will run the AC all the time (within its duty cycle), mixing heat with the cold air in the Summer to regulate temperature, and running the AC to dehumidify the heater air during the Winter.