The standard tire on the H and M was 38". I have an original brochure that states 10-38 for H and 11-8 for M. I can't speak for the factory putting on 36" tires, but I wouldn't be surprised if some of the steel-wheeled tractors were converted to 36, because this was a very common tire size in the late 30s.
The F-20 usually had 36" rims on rubber, and I definitely remember F-12s with 36," point being that this was a commonly available tire. My father's 1938 F-12 came from the factor with 38" rears, and I do think that was the most common size for this tractor. From my original IH brochure for H and M: At 1650 full-load rpm for the H:
4th 5-3/8 mph 5th 15-5/8 At 1450 full-load rpm for the M:
4th 5-1/8 5th 16-1/4
The brochure does not give the tire size, and it does not actually state that the speeds given for gears 1-4 are on rubber. It does make it clear that fifth is available only on rubber (you probably know that fifth was "installed" just by turning out a little bolt in the transmission cover--when turned in, it blocked the shifter rail from moving forward into 5th!).
The no-load speed for H is 1815 rpm and for M is 1595 rpm. In both cases, this is 1.1 times the full-load speed. You can multiply the full-load ground speeds by 1.1 and probably come pretty close to the no-load speed.
What others say here about the huge gap between the gears is right. The H, especially, desperately needed another gear between 4 and 5. On a gravel road with a wagon-load of hay or grain, the tractor would not pull the load at 5 or 6 mph in fifth, and in fourth, you got under 6 mph with the engine roaring away like a race car.
Most of the tractors of that era were later re-tired with wider treads, for better traction, and there may be some slight variations in ground speeds because of that.
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