If you want to make sure they are the stock size, check the vehicle ID sticker located below the door latch on the drivers door when the door is open. This sticker will tell you the proper size and inflation pressure. I believe 235/85/R16 Load Range E is the proper size for your dually but that sticker will tell you for sure. If that is the proper size you won't have any problems with the tires rubbing together on the dual rears or during any type of turn or suspension flex.