18" tyres

[motto (RIP)]

You're perfectly correct Lynn, tyres of the same nominal size from different makers can vary significantly in outside diameter. As you and Grant point out this also applies to different types of tyres (run flat, cross country etc) from the same maker.
However, it's the diameter of the hole in the middle that I was rambling on about. The topic that has been covered previously I know but there are those more recent victims of the OD bug that have yet to encounter this 16" wheel/tyre anomaly.
Bead angle aside, tyres made for wartime British 16" wheels are oversize enough to be quite loose on an American wheel as I proved again a couple of years ago when I inadvertently purchased the wrong ones.
I know that in the past some people have welded steel plates to the wheel bead area to centralise British tyres on Dodge WC wheels otherwise they could end up quite eccentric. I myself have in times gone by bonded rubber strips to the tyre bead for the same purpose.
Very little of this has to do with 18" tyres so once again I'm guilty of hijack. It just seems to happen in conversation.

Cheers

David