Stock offset is 22 on the rear. A 9.5" rim 20 offset will protrude an additional 8.35 mm towards the inside if you keep the stock 255 tire. My setup with a 285 tire (that rubs a little) protrudes 12 mm (approx) more than stock to the inside and 18 mm (approx) more to the outside. What tire do you intend using? A 20 mm offset and 265 will be equivalent to what I have on the inside. (that rubs a little/lowered)

Les