http://babelfish.altavista.com/:
"COUNT OF DIMENSIONS OF THE TIRES
How to determine the size of a tire?
There are 3 denominations: Metric, Agrarian and US.
A 215/70R15 is a radial-ply tyre 215 mm broad (R) with a side a height of 70% of its width is 150 mm, assembled on a rim of 15 inches (1 inch = 25,4 mm) of diameter. If the height of side is not indicated, 205R16 for example, it thus acts owing to lack of a height of 80% 205R26 = 205/80R16.
A 750x16 is tire of which the height of the side is 7,5 inches is 190 mm assembled on a rim 16 inches in diameter.
A 31x10,5R15 is a tire whose diameter external of 31 inches, of 10,5 inches broad, is assembled on a rim 15 inches in diameter."
--
Antti R.
1978 HJ-45LP