Backspacing vs Offset o

[Davissw1]

I have a 66 coupe and was looking at getting some of cruzin concepts wheels in 17x8. But when I called the guy asked what offset and I told him I need 4.75 backspacing and I was going to run 235/45/17 tires. Now I have a confusion between backspacing and offset. If I want 4.75 offset would that be 6 mm or 8mm?

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[Kotobuki]

Backspacing is the distance from the inside rim to the inner mounting surface of the wheel, measured in inches. Offset is the distance of the mounting surface from the center of the wheel, measured in mm with (I believe) positive numbers meaning the hub is offset toward the outside of the car, and negative being offset toward the inside of the car.

According to this site, my numbers are not quite right (which I've removed to avoid confusion)...

And they say it's 6mm, according to their chart.



And a simple formula from Mickey Thompson for doing the math yourself...

Backspacing – (Wheel Width + 1)/2 * 25.4 = offset

4.75 - (8 + 1)/2 * 25.4 = 6.35 (rounded to 6)