Are the axles and ring/pinion gearing still the only things that change on the heavier service medium duties than the regular super duties? Or is there a transmission change, or gearing change within the same case, in the bigger trucks?

The more power you generate the more fuel, air, and cooling capacity you need. I'd be curious as to what the limits are of the 6.7 also. Cooling would be pretty major - not sure if all of the extra cooling capacity would fit in the trucks or not. As pbruckne has alluded to in other posts - something has to give. What will be first?

That would be fun to be on the R/D team that pushes the limits on these engines to find where they do fail and how. I wonder how many engines Ford has blown up to find that out...