The modern equivalent of the original 10.5 X 16 tyres as used in WW2 is 262/75/R16 (10.5' X 25mm = 262mm). In other words, the tyre is 262mm wide and the chord is 75% of that = 196mm deep. Also the rim at 16" X 25mm = 400mm. Therefore the diameter must be 196mm + 196mm + 400mm = 792mm or 31.68 inches.
Thats the arithmetic, but we found that modern 262/75/R16 tyres looked too small on our LRDG truck (based on WW2 photos) and we had to go another size up before we got a tyre diameter that "looked right".
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