Before the 190/55 tire profile was common, a popular modification for literbikes with OEM 190/50 rear tires was a swap to 180/55 rear, with some of the benefits you mention in your question. On your Hayabusa, replacing the stock 190/50 rear tire with a 190/55 will raise rear ride height by the difference in radius, 8mm in your case. That’s a pretty significant change, enough that you would feel quicker and lighter steering. Gearing will change in proportion to the circumference, about 2.5 percent or equivalent to a one-tooth change in the rear sprocket. This you would not notice so much, especially if the majority of your riding is on the street. As for the differences between the 180/55 and 190/50, in our last tire test (“Both Sides of the Fence,” April ‘09) we saw a difference of just half a pound between the Bridgestone BT-016 in those two sizes, and even though that weight is at the very edge of the circumference, again I don’t think you’d really notice that. Note, though, that among other brands we’ve seen as much as a two-pound difference between the sizes, a fair amount. Even though the 180/55 is taller than the 190/50, the wider rear tire typically has more actual tread width as well, and I doubt the 180 puts down “a lot more rubber” at the lean angles you’d typically encounter on the street. The biggest fitment concern when fitting a taller tire is clearance to the swingarm pivot area. Given all the above, I think in your case the 190/55 would be a better option. In the Bridgestones, the difference in weight is negligible, and the Hayabusa certainly could use lighter steering from the taller profile.