I had access to a 21x2.15, 21x3, and 21x3.5 rim all with a 90/21 tires on tehm yesterday and snapped a few pics. It seems that the 21x3 and 21x3.5 are almost exactly the same width with the 90/21 tire on them. I measured and it was about 1/4" wider on the 21x3.5 only. I did not measure the 21x2.15 but it was a little narrower. It looks much narrower in the pic due to the fender as it is on a night train.
Night Train 21x2.15 with Avon 90/21


Heritage with 21x3 Avon 90/21


Fatboy with 21x305 and Avon 90/21


Drew

 

Drew , The tire looks good on the Train but to narrow on the Fatboy. The wide fender needs a wide tire. My opinion. Here is a pic of a 120/70-21 Vee Rubber on a 3.5" Mammoth on my Lux. Fills the fender good.
Stone

 

What happened in the pic of Tony's bike? The quality is about 10% of the other two ;^)

 

I dunno. Iphone camera...

Drew