Some tyre 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 tyre widths. Different tyre manufacturers specify different rim widths for their tyres.
From a warranty/insurance viewpoint, I can only offer that in the 1995 916 owner's manual, Ducati specified the 180/55 as an "alternative" to the 190/50. Also, the bike's under-seat specification sticker also listed both sizes as recommended.
[Edited on 5-10-2004 by Shazaam!]
Shazaam!
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