How do you convert mm offset to inch backspacing?

[SickSpeedMonte]

Yep...it's magic. It's called math.

If you don't want to use an online calculator, just convert your offset dimension to be consistent units, inches in this case, 25.4mm = 1 in, then add half of the wheel width if it is a positive offset and you've got the backspacing dimension.

For example, 35mm = (35mm)/(25.4mm/in) = 1.378 in
1/2 Wheel Width = (.5)*(9.5 in) = 4.75 in
BS = (4.75 in) + (1.378 in) = 6.128 in

Dont you also have to add the width of the wheel "lip" (that the tire seats on)? Usually about 1/4"



The rim width is measured from inside of that lip, where as backspacing is measured to the outside.

http://home.earthlink.net/~bzdel/TireWheel.htm