Offset and backspacing calculator and questions

[Fosters]

Just wanted to share this, and ask some questions about it, found this offset and backspacing calculator:

http://www.autolounge.net/calculators/tirespacing.html

By my calculation; our wheels have closer to 60mm offset. However, plugging in the values to the second calculator on the page, 8" width and 6.295" backspacing, it spits out 45.5 offset. Is that right?

http://www.gdmjoe.com/frppmmrim.jpg

The way I understood it, if you subtract half the wheel width (the centerline) from the backspacing, you'd get the offset:

http://www.partsprovider.net/images/wheel-offset-backspace.gif

Is that wrong? If it's right, then our wheels would be 2.295 inches of offset, which would be 58.293mm...

[Joe Walsh]

An 8" rim is actually 9" wide overall.
(8" rim width is measured inside the lip, where the tire bead seats.)

I measured my Marauder wheel's backspacing at 6.5" = +50 mm offset.

1/2 of 9" = 4.5" = 114.3 mm centerline of rim
6.5" backspacing = 165.1 mm backspacing

165.1 mm - 114.3 mm = +50.8 mm offset

[Fosters]

Ahhh, I see. Well, that's a wee better, opens up some more options :D