Width will not effect the speedometer, sidewall height will.
225 is a good size for a 7" rim, you could probably pull off 245's, I think people have even stretched 255's on, but that's pushing it, and you will probably only see a loss in performance when putting a tire on a rim that it is not suited for.
225 is a good size for a 7" rim, you could probably pull off 245's, I think people have even stretched 255's on, but that's pushing it, and you will probably only see a loss in performance when putting a tire on a rim that it is not suited for.