Originally Posted by WKdeuce
That's what I was saying. Offset and Backspacing will never add up unless you know the dimension of the rim bead. Width and Offset are measured inside the bead, Backspacing is measured outside the bead.
That is exactly correct. You can calculate offset by measuring the entire width of the wheel (outside of bead to outside of bead) and measuring the backspace. I just bought wheels last weekend and measured backspace by laying the wheel flat with the back side up, placing a flat board across the bead, and measuring from the mounting face to the board.
backspace - (outside wheel width/2) = offset
If the backspace is less than half the wheel width, that means the mounting face is closer to the inside of the wheel that the center point, and that yields a negative offset.
My measurements and calculations showed the offset was 35 mm, when the actual (according to the wheel manufacturer) was 36 mm.