[mikeayles]

Correct, the engines will have a peak reported torque at one particular RPM, usually quite low (maybe 900RPM off the top of my head), at higher RPM, the peak torque may only be 90% of the peak torque, but at 50% more RPM, you end up with (100 * 0.9 * 1.5)=35% more 'power' despite the torque being lower.

This is detailed in J1939 (canbus protocol) PGN (parameter group number) EC1 (engine configuration 1) if you fancy a rabbit hole.

The vertical clusters will be a combination of gearing and cruise control, exactly. At the vehicle speed limit in the top gear (usually 12) most trucks will cruise around 1200rpm (exception being the scanias with supercruise). It's likely this truck did a lot of urban driving or was in average speed zones.

You also get a lot of drivers (suprisingly more younger than older drivers) that are happy to set it at 53mph, get the green tick for the telemetry to try and get a bonus, but then have a really low stress drive. It creates a bigger speed differential so they can be overtaken by other trucks easier, and realistically results in 10-20mins over a 10hr shift, to some, a 10hr chilled out shift is better than 9hr40mins of pushing it.