Has anyone changed the wheels on their 1996 fatboy to 18"? If so what is the widest width tire you can fit on the rear. The wheel is a 4.25x18 rear, 3.5x18 front.
There is a post in the Wheels section of this forum which holds a chart of the widest rim & Tire combination possible without frame modification, you might want to check this out:
Data I found only goes back to 2000, but on that model an 18" tire is possible.
Best way to check is to measure your clearance from from the belt until the frame (or anything else which can get in the way like your fender). and substract a couple (3-4) cm from that. It should give you a good indication of how wide you can go.
If you have any doubts, a quick stop at the dealer might also help!