any tire company will recommend 6 - 7" wide rims for a 30x9.50 tire. and it has to do with keeping the tread flat on the ground. If you don't use the specified rim, you will either be riding on the center of the tire, if its too narrow. OR on both outer edges if its too wide.

obviously you would want as much of the tread on the ground as much as possible for traction.

Sportage sold.........
1999 4runner 4x4 , Tundra brake rotors and calipers (well worth it!) -wifes