The compressor makes the same amount of cold air regardless of the fan speed. When that pulley is turning you are loosing mpg. If the fan is on low, much of the cooling power of the compressor just dissapates, rather than being blown into the cabin. If the fan is on high, more of that cooling winds up in the car instead of blowing away with the wind.
you sort of have to think of it as two separate things. the compressor is run off of the accessory belts from the crankshaft. if that is on, then there is a direct mechanical load on your engine so you have to use some of the engine's energy or power to run the compressor. if the engine is off, then you can't run the compressor.
the fan is run off of the electrical system which is run primarily from your alternator. your alternator generates the the electricity for your accessories, like your fan (also, radio, electronics, lights, power locks, windows, etc.) your alternator i think is also on the same accessory belt as the compressor, but you don't really have any control over that except for engine speed (rpm). it is on when the engine is on.
[This message has been edited by mpeng168 (edited 08-19-2002).]