Here's the way I go about this.
8" wide OEM wheel has a 245/45/xR-17. 245mm wide = 9.645" wide at the sidewall. So that's 1.6" wider than the wheel. For optimum handling per wheel width, this is as much width as you need. Above or below this the handling/wear will suffer somewhat.
275mm/45/xR-17 at the sidewall (275/25.4) = 10.827". This puts you at 2.8" wider than the 8" wheel width. With this you really should use a 9" rim. And so on, and so on.
10" rim, go for a 295/45/xR-17.
10.5" rim, go for a 305/45/xR-17
11" rim, go for a 315/45/xR-17
Another tidbit: The wider and the larger the footprint/contact patch on the ground the lower the lbs. per sq. inch on that footprint. You can gain grip with a wider tire, but go too wide and you wil start to loose grip. They still look cool though!
GT