Improve your driving technique.
That obviously includes many things, but chief among them:
- Learn, and make use of, the "progressive shifting" technique. (Basically, shift up at the lowest RPM possible, and make your downshift at the lowest RPM possible, to lower your average overall RPM's. Lower RPM's, almost without exception, mean lower fuel consumption.)
- Use the cruise control as much as safely possible.
- Stay out of the turbo as much as possible. (accelerate more slowly)
- Keep your highway speeds and RPM's down. (If you can keep your cruising RPM's below 15-1600 and your speed below 60 MPH/100KPH, or even better, below 55 MPH/90KPH, the benefits, particularly improved fuel mileage, are astounding.
