I've got a 2005 night train fxstb with the 16x3 wheel in the back, what's the biggest wheel and tire you can run on the back without any modification? I've heard a 18x4.25 with a 160/60/18 tire...

 

I have an '02, and I'm currently trying to get my new 180 to work. I know I couldn't go larger than 150 with stock fender.

 

You two should get together, both being in Chicago! Your stock wheel will take a 150/80, if that is not stock size, but that 160 you mention is very close to the internal width of a stock fender and probably too close for comfort. To go wider may require a kit, but the chaps in the Softail section will have already sussed this out, so I suggest you pop in to see what they are doing!