The tire manufacturer knows the best how wide you can go. Choose the tire you want and go to the website or tire rack and look up the specs, it will tell you the minimum and maximum recommended rim width for a given tire size. As a general rule, the more sidewall a tire has the narrower a rim you can use. My former truck came factory with 275/55/20's on 8.5" wide wheels. And to prove my point, the 17" wheels on a SN-95 came with 245"s max. Shorter sidewalls have less flex and are harder to mount and dismount, so generally a 245 is about as wide as you can go on a 8" wheel for a car tire with a diameter of about 27-27.5", a much taller tire like a 32" has a lot more side wall, that is how they can mount a 275 wide tire on a 8.5" wheel. Years ago I was able to get a 315 tire on a 8.5" wheel, but it was a 35" tall tire.