Honestly, I think each mod I've done other than gears and wheels have improved my overall mpgs. I can still come close to the factory rated fuel economy despite having 3.73 gears. Like others said, since the car's personality changes, it changes how you drive it. That is why most people report lower numbers. However, if you remain disciplined you can still do very well. I haven't put headers on yet, but do have intake, TB, full catback and a tune. My last highway drive netted 29.06mpg.