On the trip home from Seattle when I bought the car I got 35 MPG while cruising 75-80 MPH which included several mountain passes and very hilly terrain (we get that on the left coast )

The biggest influence on fuel economy is driving style, as mentioned earlier. Better air filtration (K&N's) and better exhaust flow can give better fuel mileage, but usually the power increase they give causes more use of the loud pedal, thereby defeating any gains they produce.