That's because the US Tire and Rim Association has for some unknown reason chosen to adopt an unusual standard, where for a nominal rim size of 16" or less, the bead seat will be the Nominal size LESS .032" (+/- .015") and for rim sizes above 16", the dimension will be the Nominal size PLUS .188" (+/- .015"). The British seem to use the PLUS .188" for all rims sizes, and thus we don't ave an issue with incompatible tyres for the 20" rims.
Post-war British rims have been brought into line with the US dimensions, eliminating problems for Ferret and Land Rover owners.
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