blakamin: Just looking at the recommendations for my car and they stick a 255/55r16 on a 7.5j rim! And when you get to 18's they'll put a 255/45 on a 7.5j, so who knows?? (Mind you, you can get factory 20s for mine, as it has a massive rolling diameter, and they don't look stupid)
Thanks to meeknet.co.uk
I think the bigger the sidewall the wider the tyre can be for any given rim (just a guess - but makes sense). Although, I did a bit more looking and some Audi A4 and S4s came stock with 235/45R18 on 18x7.5 rims.