Backspacing and Offset - the never ending headache of wheel shopping
For fun I just looked up a Cragar Ss. Backspace was in inches and the offset was in metric. LMFAONot true at all. Offset is used by pretty much ALL wheel manufacturers, and the offset is usually what you'll find stamped or cast into the wheel. For example, this is a 17x9 Legend 5 wheel with a 5" backspace, 0 offset. The "0" stamp didn't come through completely, but under "ET" it has part of a 0 stamped in the box. ET is offset.
Outside of the classic car world, offset it used predominantly. Backspace is easier to measure, but if you know the width of the wheel and the backspace you can just use basic arithmetic to get the offset.
The nice thing about the offset is that it always references the center line of the wheel, so when changing wheel sizes it can be a good reference in keeping the centerline in the same place.