They are crude (but pretty good for most alloy wheels) formulas to solve either for offset when backspacing and advertised wheel width are known, or for backspacing when you know the offset and advertised wheel width.

You need the added 1" because the offset measurement doesn't care what it is, but backspacing does include the thickness of the inside flange. It's an approximation - OE Ford wheels have at least one flange that's slightly thicker. But it's good enough until you're worried about sixteenths of an inch being the difference between clearing and rubbing the wheel itself. Tire clearance is another matter entirely.

The sign for the offset measurement matters.

Example 1: 18 x 8.5" wheel, 6.80" backspacing. Offset = 6.80 - (8.5 + 1)/2 = +2.05", or +52mm

Example 2 18 x 8.5" wheel, 4.50" backspacing. Offset = 4.50 - (8.5 + 1)/2 = -0.25", or -6mm

Example 3 18 x 9.5 wheel, +45mm offset (+1.77"). Backspacing = 1.77 + 0.5 x (9.5 + 1) = 7.02"

Example 4 18 x 9.5 wheel, -13mm offset (-.51"). Backspacing = -0.51 + 0.5 x (9.5 + 1) = 4.74"


Norm