911 tires on fuchs rims

I have a '86 Carrera coupe with fuchs 16" rims. I have 225 55 ZR16's on the rears. But everyone is telling me they should be 225 50's. Can anyone confirm this? I'm about to replace them and I'd like to get Michelin Pilots and the 50's are considerably cheaper than the 55's ($130 vs $228).

My '84 Carrera 3.2 (European version) has 225/50-16 on the rear and 205/55-16 up front. These are mounted on Fuchs rims.

Porsche recommends (but it's law in Germany) getting tires with the "N" rating. THis is not to be confused with the speed rating (you'll probably want to get "Z" rated tires. The "N" rating is a tire compound specifiaclly for Porsche. These cost a little more. Currently, I think only N-2 is available. Bridgestone, Continental and a few others make them.