I would think the speed limit was more to do with stopping the rig at that weight, even with the extra axles and brake drums of the later Dyson trailer. That Hercules was a well engineered piece of equipment for its day, and ahead of its competitors in some respects. Both power units give a good account of themselves even today. I was involved years ago finding out what pulling power was needed to move a 115 ton trailer and load from a standing start. It turned out the initial start was 7840 lbs with 1120lbs to keep it rolling on the level. That was within the site speed limit of 10mph, then the old equation double the speed square the power is relevant. Of course the fuel that we use today is not the same as when these engines were designed.