i have a 2006 fx4 f150 super cab with 18" wheels and 275/65-18 tires on it.what is the biggest tire i can put on stock rim with no leveling or lift. and no rubbing.

 

BFG makes the AT KO and MT KM in 285/65-18 which is basically a 33x11.50-18. These will fit easily. The next size available (in other brands) is a 325/60-18 (33x13.00), but it is a little too wide for the OEM 7.5 inch wide wheels. Some people are running them on OEM wheels, but I think they all have leveling kits.