My car has an old-fashioned climate system from the 90’s, with manual controls rather than the new computerized ones they have now. It has a fan knob with several settings, but the A/C only turns on and off with a switch. (The temperature knob only adds heat, so that should always be full cold when using the AC.) I’m wondering, does the fan setting make any difference in how much fuel is used to power the AC? Do I save gas by having the fan on low rather than full blast?
Actually there is no reason not to put the temperature knob on a warmer setting. You will wind up getting warm dry air. In fact in some cars when you turn on the windshield defroster your AC actually turns on even if you have the temp setting on the warmest setting.
Turning on your fan to the high setting would have a very marginal effect on your fuel efficiency because your alternator would be under higher load and therefore the engine would have to work a little harder to keep it turning. You can’t get something from nothing after all. I don’t know that the effect would be measurable in any meaningful way. You may as well avoid driving with the lights and radio if you’re going to go to that extreme.
I know the fan itself uses little energy. The A/C compressor uses a lot. I’m wondering if changing the fan setting affects how much energy the compressor uses.
The short answer is: No.
ETA: I type too slow. I was replying directly to the OP when the other posters above beat me in there.
The fan consumes electricity. Whch must be generated by the alternator. Whch must be powered b burning fuel. So there is a fuel cost to running the fan at a higher speed. The difference is negligible in practice.
The air conditioing comprssor cycles on and off based on a thermostat or pressure switch in the refrigerant system. It is likely that a lower fan speed would be slightly less effective at transferring cool from the evaporator to the airflow. (Or more precisely, transferring heat from the airflow to the evaporator).
So a lower fan speed should cause the compressor to cycle on less frequently or for less duration. Which will consume less fuel. This effect should be larger than the fan electrical load effect, but will likewise prove all but neglible in practice. It might be detectable on a dyno; it won’t be at the gas pump.
WAG: If you’re driving over about 50 mph, slowing down 1/4 mph will probably save more gas than running the AC on low fan vs high fan.