How wide of a tire can be run on a stock rim. Ive seen 12" wide tires on 6" rims but I dont like it. Especially on a heavy diesel truck. I would like to safely run a 12.5" tire without worries. And would rather save the $$ needed to buy wheels since Im not worried about purdy rims.

 

My '89 has 6.5" rims with LT235/85R16 tires. The tire width is 10.5" loaded. According to my tire books, you go go as high as an LT255/85R16 which is 11.7" loaded width. Anything wider is not recommended for our rim size.
Ron

 

I have seen 31x10.5 tires run on 6" rims, but they wear the center of the tire out faster than the edges. I would guess you need to stay within a couple of inches of the tire width if you want maximum tire life.

 

If you want to run a 12.50 tire, you want at least an 8" rim. Personally, I prefer the 8" wheel to a 10" as the rim is protected somewhat by the sidewall of the tire.