The best mileage I have gotten was 48 mpg, and the worst 40 mpg.
On the preload go by the marks on the side of it. Five should be visible with the bottom line right at the top of the forks. Not sure how many turns to get you there but watch the marks. I have mine at 5.
Rebound is done by counting the number of turns from fully screwed in using the slotted adjuster. Same with compression as far as turns go but it is located toward the bottom of the forks. If you are wanting to return to factory settings make sure pre-load, rebound and damping are within spec for front and rebound and damping for back. Your owners manual will have the settings. You can always run by a dealer and have them set it up for your height/weight or preferences also.