as long as the tire width is the same or wider thn the rim width youre good to go. i have 12" wide tires on 7" wide rims, 8" wide tires on 6" wide, and 10" wide tires on 7" wide rims. 0 problems. but the only thing you have to remember is that the wider tire on a narrower rim will be taller.
ex. 25x10x12" tire on 12x6" wide rim will be taller than a 25x10x12" tire on a 12x8" wide rim.
-94' trx300FW:blue, warrior front shocks, 300ex rear shock, 300ex rear hubs, rincon rims, 25" executioners, cycle country boot guards, quad mag shock covers, warn 4-2-4, 3000lbs winch, k&n, cobra full system, blue T.H.E. lock on grips, 250BR cam chain, EBC clutch with spacers on springs, custom reverse lever, snork'd, 250sx cam
-85 200x: 200S motor, 1 tooth up front sprocket
-85 200x: megacycles cam, wiseco top end, custom heel guards, full cobra, k&n, irazr radial rears, trail wolf front.