Items required to measure wheel backspace:
The easiest way to measure backspace is to lay the wheel on a flat surface facedown (be sure to protect the wheel from scratches, carpet works well), with the backside facing up. Lay the straightedge across the back of the wheel and measure (with the tape measure) the distance from the axle pad (the part that contacts the car) to the straightedge. The diagram above shows a 2", 3" and 4" backspace. Front space is the direct opposite of backspace. Subtract backspace from overall wheel width to get front space measurement.