Ive got a 2000 sport, 4wd, 4 door, 2.5 inch lift, 30x9.5s stock wheels. Anyways I drive country back roads to and from work averaging 50 mph, which should yeild me awesome fuel economy...for a cherokee. However I consistently see 15-16mpg. I have 104k miles and have a feeling it may be time to...