Offset or backspace

Offset will be in millimeters, if you mean a 7" wheel with 4" of backspace, that would be because it has a 0mm offset.

Honestly, with this being written communication, I'm not sure if you're asking seriously or not. If you are, look at the chart I shared up above. It will show you what the offset and backspace will be for a wheel of a given width. If you know two of the measurements, you can find the third.

I mean what I wrote. offset and backspace do not use the same measurement for wheel width. the math proves it pretty simplistically
 
I mean what I wrote. offset and backspace do not use the same measurement for wheel width. the math proves it pretty simplistically

Ok, I see what you're getting at now (I'm cooking dinner at the same time.) Yes, in that sense they use a different width because backspace accounts for width of the lip whereas offset doesn't. However, that's pretty much just how they're labeled. It doesn't change that the chart will still answer a person's questions about offset vs. backspace.
 
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Most miss the point that a wheel's backspacing will instantly tell you if it's going to rub or not.
Offset tells you nothing unless you know how wide the wheel is (and can do math).