No it's not 10 gallons, I also have a YJ, but a 94, regardless i also have the 15gal tank, I also deal with this sometimes when filling up but i just assumed it's a jeep thing, by the way when I fill up all the way from empty it stops at ~13.8gallons
At least it's not spewing gas all over the ground like a lot of us have to deal with (youtube has videos). Every time I fill up no matter where it is I have to listen closely to it filling so I can try and let go of the handle before it spews. Sucks when the hooptie next to me has his radio blaring and I can't hear it filling.
My 06 TJ use to overfill and flood onto the ground. Took it in to the dealer 4 times and never was fixed and they replaced just about everything but the tank itself. I just had to keep my hand on the pump the whole time waiting for it to make the click just as it was going to overflow.
So, a year later, I have time to let my mechanic trace out the wiring under the dash to see whether he can inexpensively undo the madness that the previous owner did. Joy, only the main wires were spliced to hell; all replaced.
On a full tank 'o gas the gauge is pegged, as expected. It says pegged until I'm about 75 miles in. Then it seems to drop in the reasonably expected fashion, altho I haven't run it dry to see the bottom point. (I may try to do that next week in the desert.)
Searching the 'net uncovers similar behaviors for folks who have replaced the stock 15 gal with a 20 gal but not finished the tweak. I haven't checked, but assuming that this is stock like everything else, what are the most likely causes?
There is no 15 gallon tank. All tanks were made to hold 20 gallons, but the fill vent only allows 15. If you search for the gas tank vent modification, you'll understand more. Basically, you could pay the stealer for a "20 gallon" tank, and they just modified the vent and put in a different sending unit. If you do the mod and don't change the sending unit, the gauge will read full until you get below 15 gallons.