Simple enough, Jim.
All you need to know is the tread you're wanting to obtain and the wheel-hub surface measurement from side to side. Subtract the two and divide the result by two. If the desired tread is a bigger number, say 52.5" versus 50.0", the answer would be a positive 2.5". Divided by two, the offset of each wheel from center would be 1.25". With a zero offset wheel, backspacing would be 3.00" with a 6" wheel plus allowance for the bead. But in this case, back spacing would be 3.00" minus 1.25", or 1.75" to set the center of the tires on the ground at zero camber to 52.5" apart, which is track, or tread width. Does this make sense?
