When I fit my R1 wheel it had a Dunlop Qualifier 2 on it, a 190/50. It was really wide, around 205. When I replaced it with a Michelin PR4 190/55 it looked decidedly narrow in comparison. Not "wrong size" narrow but enough to make me check the sidewall to double check they were 190's...
A 5.5" rim is the correct width for a 180 although you can fit wider/narrower of course. You can't, for example, have a 5.5" off a different bike that needs a 190 etc. If it is 5.5" then the problem can only be the tyre is what I am saying really.
I have a std fazer rear wheel kicking about in the garage...