Sorry to hi-jack but im having the same problem and didn't want to waste another thread for the same topic.
Anyways Its a 92xj 4.0 aw4-242. Tires are 235x75x15's if that helps. Also it has 138,000miles, just replaced crankshaft position sensor, map sensor, coil, plugs, wires. Also recently replaced the fuel pump and fuel filter. Air filter appears to be clean and i just had it inspected about a year ago and it passed emissions so i figured the o2 sensor was ok. The exhausts leaves of a rich scent to it.
Just checked my mileage yesterday and got about 110 miles/ out of 12 gallons which is around 10 mpg. This is a substancial drop of mpg whenever i first got it. I was averaging around 16. I'll be glad to reply with any other information that may be useful to get my mpg's back up to where they were.