Better late than never: I came across a 90 XJ last March; 17 mpg, but now 22 to 25 mpg depending on highway grades. I ran the following, which is basicly a 90,000 mile maintenance schedule;
Tuned it up,replaced radiator hoses, flushed system, adapted an auxilorary fan switch, lubricated chassis components, new oxygen sensor, new EGR, Changed rear and front diff fluids, changed transfer case and trannie fluids, added an Optima yellow top battery and upgraded my electrical system to run 100 amps (it came with a 150 amp alternator), changed fuel filter, replaced CCV and added a "burp cup" to catch blowby, cleaned crankcase cover inside and out, cleaned TBI, ran a ground from TPS to body. I plan to lift it and gear up to 4.10 in the future, so this mpg will change somewhat. <img src="/forums/images/graemlins/cheers.gif" alt="" />