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Originally Posted by ice745
The other thing is with the A/C I don't understand how it affects gas milage. The compressor is belt driven, and its not like the compressor itself consumes gas. Since the belt is always going to spin whether the A/C is on or not, I don't see how it can affect gas mileage. I understand the pully will only spin if the A/C or Defroster is on, but does the amount of force needed to rotate a pully really affect gas mileage? Or is there something else I'm missing.
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The difference here is the draw the AC system uses.
When you do not run the AC, the compressor pully spins around, but the compressor itself does not turn. It is mechanically disconnected from the pully.
When you turn the AC on , the compressor engages and rotates. This rotation draws power from the engine, so the engine must burn more fuel to move the car AND spin the compressor.
Using the AC on the highway is not always a bad thing. Often cars get better mileage using the AC and having the windows closed, than no AC and having windows open.
In this case, the extra aerodynamic drag of open windows is more than the drag of the compressor.