Let me preempt this by saying that my XJ will never take more than a 15 gallons, even if the gauge is buried at "E." When I fill up, the first 3/8ths of the gas gauge will drop after 20 or so miles. From there, the mileage is pretty decent. What do you guys think? Maybe my tank is registering as full when it's really not?

 

weird. I can cram about 18 gallons in mine... I think you should run it out of gas one time. (not the best for the fuel pump) But i think your gauge is messed up.

Just keep a gallon a gas with you, when you know its gonna run out. I bet it will go a couple days before it actually runs out.

 

My gauge pegs about 1/8 a tank past full whenever I fill it up. It'll click off at the pump at full, but since I usually fill it when I hit 1/2 a tank, I know I gotta pump in another few gallons. It never spills, so who knows?

 

Mine reads E when its full and moves to the right as I drive

 

Take the cluster out and reverse the wiring on back of the gauge.

 

My gauge is very random.
87 XJ with 20 gallon tank.
IT was on 1/8 tank this morning and I put 11 gallons to fill it up.
On empty I don't think I've ever put over 17 gallons in it.
If I fill it up instead of the wife, it will read F, if the wife does it, she doesn't work to get the last gallon in there and it reads slightly more than 3/4.

 

Its not actually the guage thats doing it its the junkyard pump that I just put in. Either way not really worth the effort IMO