I dunno... my 89 MJ 4.0-5spd @ 100,000 mi (33"-3.07:1 then 33"-4.56 @ 3-5") and a leaking exhaust gasket got around 18 MPG either way. After swapping in a stock rebuilt motor and a new gasket it got about the same...
My 89 XJ beater 4.0-AW4 auto @200,000 stayed in the 16-19 MPG range going from 31" to 33"/4" lift with 3.55, then to 33" 4.56:1 and 6.5", and then back to 31" again.
My 88 stocker 4.0-AW4 @220,000, 225/75 3.55:1, gets 18-20...
My 90 stocker 4.0-AW4 @220,000 235/75 3:55:1, got similar (IIRC)
NOT COUNTING WHEELING TRIPS, Getting less than 16 average (or beyond 20) over several 'normal' tankfulls with a similar loadout, in any of these older Jeeps, and I'd be looking for "what's wrong?"