in my 91 YJ, from bone dry empty, I can put 5 gallons in and the guage reads full, add 2 more gallons and it pegs out full tank, but is hardly even a 1/4 tank of gass! This is normal when the sending unit float has been tinkered with when having the fuel tank sending unit gasket recall done or from other activities.... My situation isn't that rare, nore is it common. Don't rely on your guage. Filler up all the way, not just when that vapor recovery thing shuts the nozzle off at the pump, all the way, gas at the mouth of the filler tube, then run her normally. If you still only get 100 miles for a tank, consider getting a locking gas cap, consider getting a new cat converter, consider fixing the leak, consider replacing the charcoal vapor bottle (its the strange tank under your w/s wiper fluid tank with 3, 4 oe 5 tubes coming out of it) or checking your odometer calibration. If not any of these things, dono........