What is the stock wheel Back Spacing? Thanks
18x9, 6x139.7 bolt pattern (also known as 6x5.5), +19 offset, 77.8 center bore, M14x1.5 lug nuts.
One other thing to keep in mind is backspacing takes into account the width of the wheel. Offset only tells you how far the mounting surface of the wheels is from the center of the wheel.
For instance, +0 offset is in the middle of a wheel. +19 offset means the mounting surface is 19mm towards the outside of the wheel (from center), thus moving more of the wheel inboard. If the offset was -19 then it would move more of the wheel towards the outside of the truck.
Since backspacing takes into consideration the width of the wheel, it only tells you how much of the wheel is inboard or outboard from center. On a 9" wide wheel with +0 offset you have 4.5" of backspacing. On a 10" wide wheel with +0 offset it would leave 5" of backspacing. When you start to change the offset it will start to change the backspacing, but you still have to figure in the width of the wheel. There are several calculators online that do the math for you if you want to look at options.
Generally speaking, this is what I have found:
+18 to +20 offset on a 9" wide wheel you can run 37" tires with no suspension mods.
-12 offset on a 9" wide wheel will require a level kit to prevent rubbing with a 37" tire(and it may still rub a touch depending on your tire choice).
Any offset between those two options will allow some compromise between tire height, width, and need for suspension modifications. Again, tire choice can impact the results since not all tires are the same height and width in a given spec.
If you want to run 35" tires, you can start to push the offset further negative than -12 (more wheel sticking out from the fender). I don't have any exact numbers, but I think some guys are running -18 and even more with 35s without rubbing.