If some of this looks familiar, it's because I posted parts of it to another Ford forum recently. It refers to wheels fitted to a late 70s MkII Escort, but hopefully the following examples will still be of assistance to those with other cars.
The 13 x 5" 'styled' steel wheel (as fitted to GLs and the like) has about 102mm backspace (i.e. 4"), making the offset about 25p (i.e +25). The offset isn't stamped on the wheel, so this value is a very close approximate.
The 13 x 5.5" alloy 'Volante' wheel, which was optional on all Escorts but in my experience mostly seen on RS2000s, has a 108mm (4.25") backspace, making the offset about 25p (same disclaimer as above). It makes sense that Ford did this, as it keeps the vehicle track exactly the same as the lesser steel wheel.
To achieve the same 4.25" backspace in a 6" wheel requires a +19 offset, which, probably not coincidentally, is what many UK RS 4 spoke alloys came with, and for a 7" wheel the offset is +6. Some people have fitted 7" wheels with a +6 offset to a Mk II and found that they have had to roll the guards, whereas some with 7" and offset +12 have said they have no such problems (as they are set back a further 6mm).
I suggest determining how much backspace you have and working backwards from there. Remember, the offset is a measure of how far the mounting face is from the center of the rim, which is why the number changes with rim width even though the backspace might be the same.
post edited by Flighter - 2012/04/12 18:00:15