Originally Posted by starquestbd22
-18 offset should be plenty enough to clear 12.50's without rubbing. If you stick with a 9" rim you are basically moving the rim and tire out from under the truck about 1.4". That combined with the 12.50" wide tire will definitely make it stick out. As for the backspacing....it's basically a product of the offset and width. If you already know the offset and width that you want, then you don't need to worry about the backspacing. If you stick with a -18 offset then the only way to change the backspacing is to change the rim width. Make sense? Hope this helps.
To further explain what this guy said... A -18 offset on a 9" wide rim gives you a backspacing of +/- 4.25". A -18 offset on a 10" wide rim gives you a backspacing of +/- 4.75". From what I've read on forums you can get away with a 5" backspacing with 35x12.5 tires and a leveling kit with no rubbing (possibly some at full lock). So with either of those wheels rubbing shouldn't be an issue. It just depends on how far you want your tires to stick out of the truck. Here are two good sites I found to help you visualize the size of each: