18" tyres

[motto (RIP)]

My suggestion of tyre designation 9.00-16.25 was based on the assumption that the HUP and the Humber 4x4 used the same wheel diameter as other WW2 British designed military vehicles with 16" wheels. These were about 1/4" larger in diameter than 16" American wheels.
Strangely enough the post war Ferret Scout Car used the smaller American wheel diameter. The run flat tyres for these were marked 'To fit 6.50 H Rims' if I remember correctly.
There are those who think the difference in wheel diameter is insignificant but I have tried putting tyres made for British rims on a Dodge WC and they were definitely a sloppy fit. Conversely, I know of at least one fellow that had his Blitz wheels machined to a smaller diameter to take American tyres. He was happy with the remaining thickness.

David