First, measure the overall width of the wheel (remember, just because a wheel is 18x7.5, does not mean that the OVERALL width is 7.5”. It means that the measurement between the outboard flange and the inboard flange is 7.5”). Next, divide that width of the wheel by two; this will give you the centerline of the wheel.
Overall width/2 = Centerline
After determining the centerline, measure from the hub-mounting surface of the hub to the edge of the inboard flange (if the wheel were laying flat on the ground – face up – your measurement would be from the ground to the hub-mounting surface). This is your back spacing.
Back spacing - Centerline = Offset