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You are limited to 14gal. on the stock '89 tank. The newer plastic tanks, '91 and newer I believe, used the same plastic tank for both 15 and 20 gallons. Only difference, the 15gal model had a tube inside the filler hose that extended about 7 inches into the tank, preventing fillup to 20gal. I trimmed this tube on mine, and can now hold 20+gal, really sweet to extend the range from 200 miles to 300 miles.
You best bet would be to find a used 20gal tank for your year Jeep, or at least '87 to '90, they should be the same.[addsig]