Devon: I think a lot depends on what you plan to use the tractor for. 12.4 and 13.6 are dark ages tires for that much tractor. I farmed for a number of years and was always disappointed with 15.5x38 tires. My 560 and 656 came new with 15.5 and they were good on roadways and hard ground, but they didn't have any better drawbar clearence in the hay field than my 300 on 13.6. The low profile gives very little more ground clearence than a 13.6. Personally, if I were using the tractor for farming I'd go 14.9, about an inch higher profile than the 15.5 and it will give better drawbar clearence and pull better in the field. If I were going pulling I'd buy the 15.5. You just take a look at what is coming on modern day big tractors, 42 and 46 inch tires. The farming industry is catching on tire circumference is better than tire width in the field. It will rob your power, but these new tractors have extra there for pto work. My dad saw this in 1951, had been farming with a W4 on probably 13x26 tires, bought a new H with 11x38. In the field, H was all over the W4, however the W4 had the edge hauling loads to the barn on hard roads. I eventually went with 16.9 on my 560 and 656, and that one is a big jump, they do zap power, zap a bit too much for a 400 I would think.
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