18's with 275s on a Duster

33IMP said
What i like about backspace is that it can be measured with a tape measure and a straight edge. And i dont even have to know how wide the wheel is.
I have several wide wheels with very wide tires, that makes it hard to tell if they are 12" 13" or 14"wide. Without knowing the width, how do you calculate the offset?
If i look at my wheel, and one has 5 1/2 bs, and the other has 3 1/2, i know which one will fit on my car.
Tell me you think that wheel is a 14", you think, and you think the offset is +50mm, or whatever, i have no clue whether that will fit.

First, if you can't tell how wide the wheel is then you don't know if it will fit with only the backspace measurement, because you don't know how far it will stick out. You still need more information. If you have that information, then you can come up with the offset.

Meanwhile, if you tell me the offset I can tell you exactly whether or not it will fit. You're just not understanding how to use offset, and you don't use it enough to be familiar with it.

And finally, the offset is cast directly into most modern wheels. So, you just need to look at the backside of the wheel for the "ET" number. Backspace usually is not and has to be measured.

Like this

Or this (not the best picture or stamping, its a 0 )