245's are generally run on 8-8.5" rims. 275's are 9.5-10" rims.
A 7.5" rim should run a 225, although you can get away with a 235.
Keep in mind if you put say a 245 on a 7.5" rim, it won't have the same contact patch that it would have on an 8 or 8.5" rim. Some people stuff 275's on the stock 8" GTO rim, FWIW. I still wouldn't do it.