I was told when I bought my Ford B-800 from the used truck dealer that a school bus usually has a modified system so it can only go up to about 60mph (or the throttle is otherwise limited). Going up a hill, it is SLOW, probably about 20mph at some times. The engine is a Cummins 5.9L.
Is there something perhaps to adjust with the throttle levers? Or something with the injection system? The dealer said something about "plates" that potentially block something. I really don't know much else.
Of course going any faster (speed or acceleration) is maybe a dumb idea when the vehicle fully loaded is around 10-12 tons.