Stock rear rim sizes for the 550 are 1.85 inches front and rear.
A 120 on the rear will work ok when new, but will square off much earlier than the proper size tire and lead to undesirable handling qualities. Thus they wear out faster because you have to run higher tire pressures to compensate for lack of sidewall support from the narrow rim.
After trying oversize tires on my 550s for several years, I went back to near stock sizes. 3.25 Metzler fronts, and 4.00 Metzler rears.
They adhere well enough to scrape the pegs. And, they have restored the nice handling characteristics the bike had with stock tire sizes. I'm getting better tire mileage, too. But, haven't yet worn out a set.
I'll keep buying Metzler till they don't make the right size anymore.
Cheers,