4.5" has been a safe bet for me.. Keeps the tires off the control arms and off the body. Although with 4.5" on a 10" wheel you get very close to the body if you run bigger then a 33x12.5 you would need longer control arms. Can also run up to a 5.5" to keep the tires tucked in a bit more and not have to worry about flares. Not sure how the 10" wide wheels do with that backspacing though.