If I understood it correctly (read the sticky already)....

I rounded the #s off.......

The wheels I'm looking at...

18" x 8"-40mm offset=5.5" Back Spacing?
or
18" x 9"-45mm offset=6.25" Back Spacing?

Is that correct? I was wanting the 9" ones, but 6.25" is too much BS. 8" BS just isn't worth buying. I'd go wider than 9", but that's what's available, in the design I want.

 

Oh yeah....

In the sticky....

shouldn't 0 offset be at a wheel centerline? Meaning for a 10" wheel, 5" is 0 offset? But the table is showing 5.5" for a 10" rim. Why is there no 0 for 8.5" width?

 

There's approximately a 1/2" lip beyond the wheel width on each side. So your 10" rim is actually ~11" wide edge to edge. That puts the wheel centerline at 5.5" from the edge of the rim where the backspacing is measured from.

8.5+.5+.5=9.5/2=4.75
No 4.75 on that table.
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Oh and while we're at it, the tire bulge past the rim lip varies depending on the tire itself. Its going to fall between 1/4" and up to 3/4" for most of what people here will use as far as street tires go. So you should add that to your backspacing or the tire will rub... but the rim wont.