i have 3.1/2 backspacing. the stock is 6 inches. i like mine with 35s i had 1.5 inches of tread past the wheel wells

 

Also just something to keep in mind when buying offset rims. They should tell you the offset in mm. A NEGATIVE mm offset will move the tire out from the truck, and a POSITIVE mm offset will pull the tire into the truck, also depending on rim width, but that a nice rule of thumb.

 

Offset is unimportant. It is a calculated value. Backspacing is what is actually important.

Offset is simply the distance the mounting face is from the CENTER of the rim. Backspacing is what determines how far out your wheels stick from the openings.