9x6/8 rear tyre size.. 190/50 or 180/55

Seriously, 190/50's should be illegal on a sportsbike, if you are found with one you should immediately be banished to the Harley corner.
Ben, just wondering what you mean by asking if "190/50 is the correct size"?
If you want something OEM to put in a glass case then there is no arguing with what you posted from the manual.
If you mean something else, ie "better handling", "better grip", "more agility" then that's a whole different question.....and the correct answer is "it all depends" Depends on what? What you want from the bike, and what you mean by "better".
Something that a lot of folks have a misconception about is what a 190 does on a 5.5" rim, vs a 180. Many folks just pinch the bead of an unmounted tyre and think the pointier parabola that appears is how the tyre will be when mounted: absolutely not so.
For this to happen the circumference of the tyre would have to change, but the reinforcing in the carcass makes this impossible, all they have done is distort the bead, something which is corrected once the tyre is mounted.
So the tyre profile does not get more "pointy", but it does get distorted, staying much the same in the middle but bulging more towards the edge of the tyre. Forgive my crap drawing, but something like this for say 180/60 vs 190/55 (approx same diameter) mounted on a 5.5" rim:

Supersport racers love the 190 or even 200 on a 5.5" rim because they can get a big fat contact patch like this (albeit running just 26psi in a Pirelli hot off the warmers):


That's Shakey Byrne (BSB) but you get the idea: any handling compromises are acceptable if you can get a contact patch like that, even with "only" 150hp in a SS machine.
Even if you're not Shakey slumming it on here, realistically it's not a night vs day handling difference, it's something you can largely make up for with the geometry adjustment available on a Ducati (I used 23.5deg steering head angle even on the road on my old 916).
You've just gotta ask yourself what does "better" mean for you? Is slower initial turn-in actually "worse"? Or just different? You might like slightly more agility if you're carving canyons, or maybe you want the big footprint for punching out of slow corners at a stop/start goat track. Or you might just want the extra mileage a flatter 190 centre section gives you if you're touring. "Better" is a very subjective thing.

chris_f said:

You arent wrong, if I were spending ridiculous amounts of money to eliminate weight I might go through the exercise. But at that point which is more important, a couple ounces or weight or a noticeable amount of grip? And where do you stop, titanium fasteners, aluminum sprocket that will wear out often, hell for that matter why not swap out the caliper pistons for titanium if you are going to go crazy. At some point you have to factor in diminishing returns.

Everything we do to these things is either diminishing returns or a personal preference. Hell I have heard some people complain that the 916 series bikes feel unstable when they turn in quickly because of a tire profile change. But as pointed out you could accomplish the same thing with setup.

Click to expand...

A long time ago I decided I was going to pursue lower weight on my 996 as a general philosophy. I was inspired by three things; the feel of the Honda Hawk that I owned three years before I purchased my first 996, Tadao Babas design philosophy behind the original Blade ( chase light weight for every single component obsessively because no matter how small the weight savings may be, the knock on effect to the next component, that can then be designed lighter, and the motorcycle itself , can be profound in the end result ), and the heavy, ponderous, uninspiring feel of the stock 996 when I first bought it in '03. I knew I loved the look of the 996, the feel of the engine, but in stock form it needed a fair bit of work to make it a satisfying, responsive, back road scratcher.
Of course, taken on their own in isolation some of the weight saving measures I've followed don't make much sense. But the 65 ish lbs I've lost from the stock 996 weight DOES make a lot of sense to me on a twisty road! Especially if I cast my mind back to the ponderous beasty I first purchased twenty years ago this September.
I will add that one of the first changes that made the biggest difference to handling dynamics was the change to the 180 section rear. With the 190 the bike required too much effort to steer into the corner, and then on the throttle on the way out she just wanted to head for the weeds, somewhat regardless of my best efforts to encourage her otherwise. With the 180 she was altogether a far more obliging dance partner.
With the 180/60 I didn't really notice that much of a difference to the 180/55, likely because when I mounted the 180/60 it had been a few year gap since I'd last ridden the bike with the 180/55. I might just go back to the 180/55 for the Corsa 4 just to see if i can feel any difference between the two. My rear ride height is already near maximum