I base my mileage on actual fuel used /miles driven. My 2001 3500 dually,averages 16/18 normal driving and 22 hwy. With open trailer and 3000lb.car 18to 19. With 24 both extra tall 14/15.I race at Medford 250 miles one way and do a mileage test each time and this includes some big mountains.Mine has an edge chip, unrestricted air cleaner and big exhaust.With a better tuner and other upgrades this could be improved substantially but i am not convinced that longevity isnt compremised.I know guys that pull the hills at a constant 1200 egts,I keep mine at 850 up hills.