89 YJ's came with a 20 gallon tank as an option on most models. To swap from 16 to 20 I think you must change skids too.
Some years had the same tanks (20 gal) and the 16 gal ones had a device in the top that would only allow 3/4 of a tank of fuel. It was fairly easy to remove the device.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist