Just my nagging OCD here... I understand fully how you're reaching the conclusion, but a 4" BS 8" wheel (or 4.5" BS 9" wheel) is not "centered" or "zero offset". It is counterintuitive indeed, but backspace is measured from the outer wheel lip (on the vehicle side/inside of the wheel), but wheel width is measured from the actual inner bead surface. The difference in measurement comes from the width of the outer lip (think where the hammer-on weight is installed). A "zero offset" wheel will indeed have a centered mounting surface - this is because offset is measured from the center of the wheel.
For example, an 8" wide wheel will measure 8" where the tire itself mounts - inside the bead area - but will measure almost 9" wide if you measure from outer rim lip to outer rim lip. So, an 8" wide wheel with 4" backspacing will actually have an offset of -13mm (1/2") assuming that the lip on the rim is 13mm thick.
An 8" wide wheel with 0 offset will actually have closer to 4.5" of backspacing.
And, I have no idea why inches are usually the unit for wheel width and backspacing, but millimeters are the go-to for offset.
Clear as mud?
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