Here are the spec's from the Avon website. As you see, the 90/90-19 and 3.25-19 have the same recommended minimum rim size, 1.85", which is the width of the stock 550 rims (front and rear both), and same width. The 3.25-19 is slightly larger in diameter (abt 1/2 inch) so running a 90/90 would throw your speedo off a bit, but they aren't that accurate anyway. The 100/90 technically requires a wider rim than our bikes have, but I know plenty of members use that size anyway.
size rec rim (in) alt rim (in) min/max width (mm) dia (mm)
3.25-19 2.15 1.85 - 2.50 98 664
90/90-19 2.15 1.85 - 2.50 99 647
100/90-19 2.50 2.15 - 2.75 109 671
The stock 3.25-19 front is not hard to come by (it was a common size front in the 70's and early 80's), but the 3.75-18 rear is virtually impossible to find. The 110/90-18 is an acceptable compromise. Some members recommend a 4.00-18, but its wider and larger around than the 110/90 and even though they have the same minimum rim size, I would stick with the narrower tire. Here's the specs for the 2 different rear tire options:
size rec rim (in) alt rim (in) min/max width (mm) dia (mm)
4.00-18 2.50 2.15 - 3.00 116 671
100/90-18 2.50 2.15 - 2.75 108 642
