Somehow all of the "guides" for finding back spacing and offset don't seem to make sense between their examples and their formula.

This site for example.

In their chart they list a 5.5 wheel with 3.75 back spacing as having a 12 mm offset.

They say to find the offset that you subtract centerline (5.5 / 2 = 2.75) from back spacing (3.75). Which gives us (3.75 - 2.75) 1 inch. Which converts to 25.4 mm, NOT 12 mm.

What I'm I missing?

you're missing nothing... their table is just wrong

WTF, that's the SECOND site I've seen with BS info.

Welcome to the internet

Didn't someone say, "Just because it's on the internet doesn't make it true"?


KT

Might be that the 5.5 is bead to bead and backspacing is measured to the outside of the wheel rim.Offset is also from the outer edges at least on that diagram.That might account for most of the difference.

pupwag has it right. A 5.5" wheel is 5.5" from bead to bead (rim width in their diagram). You cannot accurately determine backspace from offset by just knowing the bead-to-bead width. You need the overall wheel width. Without it, you can just approximate what the backspace is (or offset is). Which is exactly what that table does.

For the most part, I have seen the wheel flanges to be around 3/8" wide for an alloy wheel. Looks like their table assumes about 1/2" wide wheel flanges, which is where your missing ~13mm comes from.

Demick