farmall m tyres

Tyre sizes.
Imperial tractor tyre sizes were read as ge 13.50 x 32 as being 13.5" profile ( from inside of the bead to the tyre OD ) x 32 = 32" rim.
Radial tyres from memory would be read as the first number being the profile of the tyre then the next number is a percentage of the height to make the width, then the rim size. As an example if a ttyre was 400/50x24 it would have a profile of 400mm and width is 50% of the profile = 200mm and fits a 24" rim.
To get the final all important OD simply multiply the profil by 2 and add the rim diameter.

Back to front on the profile?? I think the height is a percentage of the width, so in the 2nd example above a 400/50 is 400 mm width, and the height is 200 mm, so 200 x 2 and divided by 25.4 (to get to inches) plus the rim dia of 24" gives overall dia of 39.7"

Hi Allan, that may well be correct as the radial tractor tyre was not available here when I was in the tyre game. My description was based on passenger car tyres specs.

Sorry Cobba, your still wrong. Allan has it right.

Fading memory here.

For example, when Landrover put wider rims and tyres on the Defender Xtreme wagon they deliberately chose 235R80x16 tyres, as in a pinch out bush you could put a 7.50x16 tyre on the rim and they'd have the same rolling radius.
Width of 235 mm converted to inches (=9.25"), multiplied by 80% (=7.4") to get the height, then double that (top and bottom) and add to 16" (=30.8") and it's almost exactly 7.5" by two plus 16" (=31").

From an engineering design viewpoint it also meant that they didn't have to stuff around with gear, transfer or diff ratios.