RH AO Phoenix 17″ rims. Who has experience?

Hello, I would like to have the mentioned rims mounted in 8×17 ET 35 on my 2.6 automatic/cabrio 251/1. According to the report only tyres in size 215/45 17 with a load bearing capacity of I believe 91 can be driven on this rim type 251/1. With another convertible type can be driven on the same rim 225/45 17. Who can explain this to me. Is there perhaps the possibility to drive 225 on my car? Who can help me? Greeting Achim