91 blueranger,
rubydist and racsan are correct about your tire wheel combo; though it's not the worst combination, my experience is that 3:73 is best with 28.5" and for 4:10's, 32" are perfect for mpg.
What you can do to improve fuel economy with your 4.0 is:
-lose that crankshaft driven fan)clutch fir an electric fan. One of the easiest to correct and heaviest loads these engines don't need on them.
-have a valve job performed ($150ish). I had a valve job done on my 4.0, the engine had approximately 159,000 on it and the exhaust valves were so loaded with carbon buildup, they couldn't even fully close. Also with the valve job you would be getting new valve seals, you can expect that yours are SHOT!
-the rockers and pushrods in these are known hi wear items. Dynacam of Tacoma Wa will treat you rockers and install inserts to restore them to original specs for around $150 including shipping both ways. Smith Brothers will build you a set of adjustable pushrods for just under $150 including shipping; this will permit the best possible valve train adjustment, insuring the highest possible performance. This is much trickier with the stock components as they are not adjustable.
-change your oil with that tune up ..)
That's a tune-up plus $450 extra, at some point you will need to decide how much fuel economy you need/want. I'm shooting for 30+mpg on the highway with an OHV 4.0L.
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