I think it shouldn't be a question so much as to if the gears can hold the tractor, but if the tractor can hold the load behind it. In my experience, it comes down to many variables such as terrain type, are you driving on gravel or grass or dirt, how steep of a grade, etc.
For the example, we have about 60 acres on a hill on our property that is used for soybeans/corn. The road up the hill is gravel, as to prevent erosion and ruts forming during heavy rain. During harvesting season I pull the gravity boxes with a Case IH 7120 magnum with duals. Even with this heavy machine, I still only pull one wagon at a time up and down the hill, not because it can't pull 2 wagons, but because coming back down with 2 full wagons spells trouble. Even with just the one gravity box it pushes the tractor (tires "slip" on loose gravel).
Point is, you can have a big machine that is pulling well below its max load, but depending on where you're driving, the situation can go downhill (pun) real quick.
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