Is mileage affect by the use of air conditioning

Yes, you will lose fuel economy when running your air conditioner.  However, different driving situations call for different use.  Driving down the highway at 70 MPH with the windows down for ventilation, and the air conditioning off will actually cause you to get worse fuel mileage than with the A/C on and the windows up do to the worse aerodynamics causing more drag on your vehicle.

when driving with the air con on u usually loose about 2-5 mpg if ur a christian driver and really sit at a stop sign letting every one go and then dont floor it to 60 u probably wont loose any

depends on the situation. city driving with ac on will cause loss in millage. highway drivin tho will not due to the engine already not havtn to work so hard to keep the truck movin. windows down on the highway not good idea cuz loss of areodinamics.

If you do all the math (which i haven't) you can figure the loss of horsepower, and turn this to a percentage of total horsepower, then figure how much harder the engine works because of the power loss, then figure your current gas milage and factor the extra effort and somewhere eventually you'll find out that the loss is only a percentage of a single MPG