You measure from the ball of the tire(side wall) That tells you how much clearance or distance that you can move in.Then measure the backspace of the rim that you use in the mock up.The backspace of a rim is measured using a straight edge. measure from the outer lip of the rim to the mounting surface that contacts the axle flange.When doing your math keep in mind that you need about 3/4 inch clearance between the sidewall of the tire and the wheel tub. Hope this helps 68GTS383