backspacing is how far back the wheel bolts on. So 0" backspacing, the wheel would be mounted flush with the inside of the rim. 4.5" backspacing, the wheel woudl be mounted 4.5" back from the inside of the rim. the less backspacing, the more the wheel sticks out. you will need wheel with smaller then stock backspacing so the tires do not rub on the radius arms, but this will casue them to stick out of the wheel wells