in terms of width, the 190s are the same. The 50 and 55 designations are height of sidewall. The Q2 I installed on a friend's 600f4i was a 50 or shorter tire because he didn't want to influence his gearing since his previous the 180x55 was close to the same height. Going to the 55, as i did, increases rear height thus altering geometry for quicker turn in, and raises final gearing, but is the same width as the 50.

In my case, raising the gearing was not an issue since i am running a 15/44 tooth sprockets, but on his, he would have sacrificed some low end grunt by running the taller 55.

relative to your comment of a rounder tire, it's actually the opposite. The arc of the Q2 is flatter and more uniform than i believe all other 190s on a 5.5" rim since 190s are generally intended for a 6" rim, thus spreading the bead are approx a 1/4" per side. Putting either a 190x55 or 50 on a 5.5" rim is generally going to squeeze the tire enough to steepen the sides making them prone to feeling unstable/uncertain at the edges.

The exceptions to this are the Q2 190s which are optimum on the OEM rim, and consequently the 200s are best on a 6" rim. For some reason, dunlop has chosen to separate itself from the other manufacturers and this has created a win-win for those of us with 5.5" rims who want to lift the rear, gain more contact patch and benefit from a more predictable/linear feel at lean in the case of the 55; or in the case of the 50, just want a bigger contact patch with linear/predictable feel, but do not want to alter their gearing or rear ride height.

the below reference refers to the d211, but, from my recent experiences, the same applies to the Q2s in terms of preferred tire/rim configurations.

http://dunlopracing.com/tires-products/d211gp/

Contrast dunlop with the traditional modus operandi of michelin and the other manufacturers in the chart on the link below in terms of rim/tire preferences.

http://www.gixxer.com/forums/showthr...=286257&page=2