I have been buying sets of new rims and new tyres just recently for my z and I have ended up with a bit of a bugger's muddle through taking the advice of a tyre smith.
I ended up with tyres which were the wrong size in several ways, one of them being the original rolling radius tyre size.
After several expensive mistakes, I decided to do my own research on equivalent sizes on a given rim size and I found the link below very helpful in finding the correct tyre size comparison.
Not sure this is the right section but here it is.
Hope this helps you as much as it has helped me.