Some TJs came with 15 gallon tanks but most came with the 19 gallon. It'll never take more than about 17 if you refill when the gas gauge pointer is on the E. In fact, mine only takes 17 -18 or so when it's down at the bottom of the Reserve range.
If you have a jerry can with some gas in it, you can run the tank down pretty far to see what you can put into it. I didn't have to run mine completely dry to manage an 18 gallon fill-up, so I'm pretty sure I have the 19 gallon build.
My build sheet indicates a 19 gallon tank and I've put in over 16 gallons several times. 19 gallon tanks became standard with the 1999 model year. If you haven't done so, I would suggest requesting a build sheet from Jeep/Chrysler. Got mine within 20 minutes of the online request form submittal.
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