I would think that the limiting factor on the NH35/40/45 machines would be the load capacity of the 6 ply front tires. Now that is of course with the machine weight also. I would think that the NH engineers would have calculated that with the loader capacity for the upper limit of the TC45. The TC48 which I have, comes with 8 ply front R4s, to handle the additional loader capacity. Also the TC48 front axle at the tapered down tube end is an additional 1" larger diameter.