I believe the 900ss has a 170 as standard? but it also has a wider wheel, 5.5" instead of 4.5" - not sure if fitting a 170 to a 4.5" wheel might distort it too much? As an aside I have a 180 rear on my Monster (same wheel as a 900ss) and it really is too wide for the bike as the chain rubs against the side of the tyre. It's worn about 3-4mm off the edge so I've bought a 170 to fit when the 180 is finished (nearly there), this will probably quicken up the turn-in as the 170/60 is taller and steeper than a 180/55...
