While the 180/55 isn't the normal tire, the two reasons they are used are cost, as the 180's are cheaper, and cross-section profile, as the 180/55 is a "pointier" cross-section compared to the 190/50 which is a more rounder, flatter profile.
The pointer profile of the 180/55 enhances the roll-rate response of the bike. The bike rolls into the turns much quicker.
The 190/55 tire has similar cross-section profile but has even more sidewall area.
Which is a problem I've had with Michelin Pilot Powers in 180/55.
I've ran off the shoulder going into a turn and the rear started squirming around.
Since it is a slightly taller tire, the 190/55 also raises the rear end a bit which enhances the turn-in response.
I've run all three and there is a definite difference with the 180/55 and 190/55 vs. 190/50.
Last edited by SubSailor; 04-25-2009 at 08:42 PM.