Looking to buy a 21x3.0 front and possibly a 18 x 4.25 rear if it will fit without any major mods or should I just go with the 3.5 rear wheel? Im looking to run the 150/70/18 on either rear wheel.

 

I don’t think the 4.25 wheel will work. There is just enough room for a 150 tire on a 3.5 16" or 18" wheel. 160 will go real tight.

 

150 rubs on my 06 ultra

 

my wifes 06 SG runs a 160 and it very close to the rear drive belt.

 

I have spent some time looking into wider tyres and currently have an 07 swingarm and wheel in my 1990 bike, with narrow belt and a 150/80 tyre. I can't go wider without changing the fender to a later one, but a 160 might just fit.

Actual tyre widths differ from the nominal width and also vary between brands, so we cannot expect a 150/80 to actually be 150mm wide in the bike - mine is 153mm. In addition HDF members seem to have different experiences with pushing this envelope! Some can get in a wheel/tyre X and others have been unable to, so it is difficult to be certain what you suggest will definitely work.