Maybe this will help determine backspacing. I have a 3" lift with 285/75/16 but not using stock wheels.
How to Measure a Wheel's Backspace
- Turn the wheel face (outside) down.
- Place a straightedge across the back of the rim as shown.
- Measure from the bottom of the straightedge to the center of the wheel (where the wheel is bolted to the brake drum or front hub).
Backspace is important as a measure on its own (too much backspace can lead to a wheel rubbing on suspension pieces or brake lines), but it's also important as a percentage of the total rim width. Two rims of the same nominal width but with different backspace dimensions will appear quite different on the car - the one with least backspace will yield the widest track because it will "stick out" further from the car's centerline.