article on tyre tread patterns

[Grant Bowker]

Originally Posted by Lynn Eades

The size of the tyre, 900x16, is a tyre that fits a 16 inch rim. The tyre having a 9 inch high side wall, which gives an overall nominal diameter of 9+16+9=34 inches. or in the case of a 10.50 x 16, 10.5+16+10.5= a nominal diameter of 37 inches.
I write this because I believe the article is misleading.

Period publications clearly state that the 900 in a 900x16 size tire is the "width of cross section". In most cases, tires of that period were close to being as wide as they were tall in cross section (in todays jargon a metric car tire might be 185/100-14 if it were in the same proportion). This makes the formula you gave a useful approximation of diameter but the 900 is not a specification of sidewall height.