That snippet, "not intended for over-the-road..." says it all. Doc has long had a principle, which is, if you're going to do serious trucking, you need a serious truck to do it with. Our (Class 8) truck is now 2 yrs and 4 months old, and we've just turned over 312,000 miles (team). I don't think a Class 7 truck could be expected to keep that up for too long without some fairly major expenses. And there's no way a Class 6 could. As for the different size trucks, we've done both E and D, and we can absolutely say that we make more with the D than we did with the T/T, because of the greatly-improved fuel mileage. We always get at least 10 mpg with the D, whereas we were lucky to get 7 with the T/T.
And yes, the E rate is higher, but because the average expediting load is somewhat small and light, you'd be amazed at how often the T/T is actually carrying a "C" or "D" load.