I would check the clutches for sure. I had a 99 zl 600 efi that had the hunk of cast metal come flying off @ 3500 miles. Some other zl where recalled for this but mine was not on the list. I have always had OK fuel milage. Anywhere from 11 to 14 mpg on groomed trail riding. Swapping in a 55/53 helix also will help the fuel milage.