I put a 150 Metzler on my 01 Fatboy / no problems at all.

 

I have a 2003 Fatty. I replaced my original rear wheel with a 'Deuce' wheel of the same year (2003). The 'Deuce' tire is a 160/70-17. The tire was a Dunlop and rubbed slightly. I replaced the Dunlop with a Michelin of the same size without any rubbing. You will not get a bigger tire than a 160 to fit with an original fender. BTW, the 'Deuce' wheel is wider than the Fatty wheel. Hope this helps.
Oops. I just realized this was an old thread. I'll leave it anyway.

 

So, the fender is the limiting factor? Did Harley widen the fender in 2003 and again in 2007?

 

I run a 150 and the fender is tight as well as the belt.

 

I have a 05 and a 150 Dunlop or similar or a metzler 160 (they run a little smaller than other 160) is the best you can do. The limiting factors are struts, fender, drive belt/sprocket and swingarm to go larger.

Look into a heartland kit to go larger. https://heartlandbiker.com/ http://www.chopper-kit-usa.com/fat_t...arley_FXST.htm https://viciouscycleworks.us/Category/heart

 

The FLSTF did not get a wider fender until 2006, along with other changes that allowed a 200mm tire.