18" tyres

[Grant Bowker]

I have doubts about that suggestion David.
By observation, it is considerably easier to mount modern 16 inch tires on 16 inch HUP rims than on "real" CMP 16 inch rims. If HUP rims were sized at 16.25 I don't think you would have an easy time at all getting a 16 tire onto them if it were even possible.
Original war dated 9.25-16 tires have been seen, still with original sidewall markings.
I have seen several different printed sources refer to 9.25x16 but don't recall ever seeing a reference to a 9.00-16.25 tire (they could exist but I haven't ever noticed them). The AEDB Design Record makes several references to tires 9.00-16 and 9.25-16. In all cases the 9.25-16 is listed as smaller outside diameter than 9.00-16 within category of tire (cross country pneumatic, highway pneumatic, runflat). It seems unlikely that if they were really referring to 9.00-16.25 tires they would consistently, 100% of the time, refer to them as 9.25-16 - not getting it right even once by accident.
For cars and trucks, I don't believe I've ever seen a rim diameter other than full or half inch. Tractors use a sizing system I've never taken the trouble to understand (city boy) but they don't seem to me to be a simple extension of car and truck sizing.