To Miami and back
 

I took a trip to Miami on Sunday and filled the tank up before leaving and reset the trip odometer. My 96 Aspire did pretty well round trip of 340 miles from Fort Myers to Miami South Beach and other stops. I did have the a/c on during the day and averaged 65-70 mph on the I-75, when I came back to Fort Myers and filled the tank it took only 9.190 gallons to fill. I averaged 37 mpg. My question here is if I did not run the a/c how many more miles could the Aspire have gotten per gallon? Thank You for your time.

It's hard to say how much more fuel economy you could've gotten, but the a/c is quite a drag, so it would probably be in the order of a few mpg...some others who have done highway tests can chime in here.

I'd guess A/C is in the neighbourhood of a 10% fuel economy hit.

In my experience maybe a half a mile if that. The Aspire's don't have windows that open in the rear and cause more drag than the Festiva's.. not to mention very uncomfortable. The aspire a/c doesn't seem to drag the 1.3 down as much as the festiva's with the same 1.3.

I found this piece of info on a web site about using the ac or rolling down the windows in a car here what it said.............................................. .........................
The A/C compressor pulls power from the engine wasting some gas, the effect appears to be minimal in modern cars. The A/C can lower the fuel efficiency from 10 to 20 percent depending on the type (and age) of your vehicle. Using the air conditioning does require your vehicle to burn more fuel, but it is more efficient than driving with the windows down. Putting the windows down is not a good idea. Putting the windows down tends to increase drag on most cars, canceling out any measurable gain from turning the A/C off. The increased drag of the car means your vehicle will have to work harder, because of the change in the aerodynamics. Therefore, open windows cause your car to burn more fuel.