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Tire Size VS Ground Speed, How To Cacluate?

B

Bryce Frazier

Well-known Member
I have a Farmall Super M and a M that I do the bulk of my farming with, the Super M has faster ground speeds in 3rd and 4th gear over the M.

According to TractorData, the difference between 4th gear for example is 6.8-MPH in the SM and 5.1-MPH in the M. Both tractors were tested on 12-38 tires, which I believe translates to a modern 13.6-38. Both tractors are governed to 1450RPM.

My SM has 15.5-38 (same rolling diameter as a 13.6) and the M has 13.6.

My question is... How do i calculate how much ground speed I would gain by going to a 14.9 tire? My guess is that they are a good 2 inches taller than the others. I have a decent set, but would like to know if it would be hardly worth it before going through the effort of swapping them all over!

Thanks as always, Bryce
 
RPM x difference in circumference will give you how much farther you'll go in a minute multiply by 60 for hour. 5280 feet in a mile etc.
 
Okay so at 61.6 and 64 diameter, that works out to be %3.75 taller of a tire.

%3.75 of 5.1MPH is 0.19mph, so that would bump me up to about 5.3.... Not quite worth it!

Did I do that right!? Bryce
 
I went from 15.5.38 clear to 18.4.38 and the difference wasn't hardly noticeable and and still pull everything in the same gear as before
cvphoto87187.jpg


cvphoto87188.jpg
 
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