The question might be – would you rather have more cornering grip along with a heavier turn-in and slightly lower top speed or have a quicker turn-in, less apex speed, but more speed on the straight? Getting the power and braking to the ground through the larger contact patch of a larger tire needs to be traded-off against the handling degradation experienced with a larger tire.
The size of the tire is selected by Ducati test riders during development testing of a new design. The manufacturer develops their chassis and suspension system components in combination with the tires in order to balance wear, traction, tread design, stability and handling design objectives. When you bought your bike, you paid for a lot of development and testing costs, so think carefully about what advice to take when making changes. It often will come down to individual riding styles and rider preference, so be critically honest about your own riding capabilities and needs compared to others.
Motorcycle tire sizes get larger from year-to-year so it's easy to think that bigger is better. However, chassis design development is the principal reason larger tires can be used to go fast, not the tire size by itself. So often it's best to stay with the recommended tire and rim size on a given model bike. When you modify your bike you often need to reconsider your tire needs. More horsepower generally means a larger tire is needed.
In the 1995 916 owner's manual, Ducati specified the 180/55 as an "alternative" to the 190/50 and the bike's under-seat specification sticker also listed both sizes as recommended. It wasn't too long before buyers figured out that switching from the 190/50 to the 180/55 gave a very noticeable change in cornering feel. The 180's, mainly because of their taller, steeper profile, turn-in much quicker and easier. So eventually the word spread, and everyone who has changed to the 180's has praised its positive effects on handling. That's really why the slightly lighter 748's ship from the factory with 180/55's.
A 180 tire is also slightly lighter. This will account for part of the subjective handling improvement experienced when moving from a 190 section tire. The weight difference between brands is greater, especially for the front tire. For example, 120/70 front Pirelli Supercorsa's (8 lbs. 6 oz.) Dunlop D207RR (10 lbs. 7 oz.) A 2 pound lighter tire will reduce rotational inertia by the same order of magnitude that you get when switching from an aluminum to a magnesium wheel.
Some tire profiles are more sensitive to rim width than others. Dunlop, for example, says that their 180 slick works fine with up to a 6.25 inch wide rim. In the past, both Michelin and Dunlop have stated that a 5.5 inch wheel is suitable for 160 through 180 tire widths. Different tire manufacturers specify different rim widths for their tires.
That said, it’s likely that a 190/55 tire will be marginal with respect to the chassis stiffness and require a good deal of suspension tweaking to recover the truely outstanding handling performance of a stock 748R. So the 5.5-inch rim is perhaps the better choice for most riders.
I have to admit though that the 190/55 does look bitchin'.
All the tire people i talk to seem to think that rim size isn't really an issue, as most track tires these days work the same on different rim sizes. I remember a few years ago reading an article about how mounting a 190 tire on a narrower rim designed for a 180 (i'm assuming 5.5 inch) would change the profile of the tire and adversely affect handling, now it seems that this is no longer an issue, 190's and 180's both work fine on the same rim. I have a 748 that came stock with 180's and i run 190's now mostly because i always buy tires at the track and that's usually the only size they carry. When I asked if putting a 190 on my rim would change the profile of the tire and its handling they said it didn't matter anymore. I have come to agree, i really like the 190's, they give me confidence, even if it may be false. Also, i haven't noticed the turn in being more sluggish. The pirellis are certainly more agile that the dunlops whereas the dunlops seem more stable (this is only my opinion) but both feel fine on my rims.
One caveat: I have experience with 180 street tires and 190 race tires, so obviously im partial to the race tires since they are far superior quality. I have yet to try a 180 race tire, but i can see how it would be more agile.
I guess what im trying to express with this post is that if you have the narrow rim you can still use the 190's without any trouble as far as i can tell...which makes me wonder why people use larger rims...what is the advantage of a 6 inch rim over a 5.5 when the 5.5 works fine for the 180's and 190's and even one 160's according to shazzam?
Not needed. Try and understand Shazaam's words. As I believe he is correct..
I lucked out on a set of PVM's a while ago and could have picked up either 5.75" or 6" rear, I went with the 5.75" as I did not want to deal with wider/heavier tires, that are not needed at least with 748-996's.
FYI... All the suppliers of race tires at the west coast track days, stock all compounds of 180's...
For what is worth , in my limited track experience. I had a 190 on my 999s whch had a stock 5.5 rim. When time came to replace more experienced racers pointed me to a 180 tire. I rode pretty aggressive on my 190 and did not get %100 sidewall use. Once the 180 went on it was a world of difference. Quicker drop in/lean and full use of the side wall. Both tires were pilot powers. So perhaps the comment that modern race tires don't care 5.5 vs 6 was intended for race slicks. I can't speak to that but can say it made a huge difference to the profile shape on the pilot powers.