at the risk of upsetting a good thing... i'll mention that i don't understand how some cherokees get such bad mileage.
i've got a 99 5-speed and a 2000 automatic, both with over 110,000 miles and stock except for a K&N filter.
i do not drive 'slow'... but i'm not a leadfoot either.
i'm normally the first away from a stop light, but keep at the speed limit in the city.
i'm in salt lake, which is about 4,000 feet.
to work i drive city streets with a lot of stop lights and get 17mpg average in the 2000.
when i drive to colorado springs thru wyoming, i get about 25mpg to Rawlins, and then about 20mpg to Colorado Springs. and thats doing 80 with the A/C on. i lose a little bit if i open the window and stick out my elbow.
i use the middle grade gas, which is i think 87octane.
the 5speed gets around 1-2 mpg better than the auto.
i've tried 85 and 89 octane and didn't see any difference.
i don't think i've ever even replaced any sensors... just the plugs once or twice.
one time i tried to get the 'best' mpg i could on a trip between SLC and COS, but coasting down mountains, driving behind semi's for a while etc.. and got 28mpg for the trip.. but that was to much work to do again.
i've got a 91 wrangler also and i think it gets similar mpg.
so now watch my mpg drop now that i've posted it.
i would think anything less than 17 on the highway would indicate something needs to be fixed, unless you always drive uphill or something