225/50/16 is indeed well within the golden 3% rule of variance. The size fits just fine on a (non 500) W124 but is a bit of overkill for most anything but the V8's. It does fill the fenders though so be warned. As far as rolling radius goes, it IS within acceptable (<3%) limits. If you don't mind the full fender look and want to spend the extra cash for the 16x7.5 rims that go along with the 225/50/16's its a nice handling setup provided the suspension is on par with the tires fitted. For the 6's, 205/60/15 or 215/60/15 should do just fine if given a good tire.

Lee