the fan has nothin to do with how much work your AC is doing.... it eather on or off (I am "608" and "609" certified, for all of you who don't know what that means, Its a certification by the EPA to work on AC units) and as far as gas mileage you get better MPG with the windows down if your under 53MPH and above 53MPH with AC, it all has to do with aerodynamics, witch plays the major roll in MPG above 53MPH under that it rolling resistance ie tires