Most big trucks have their engines governed to a similar horsepower; hence the similar speeds between them. Going fast downhill is a compromise: saving wear on the brakes and risking a speeding ticket versus safety factors. The ‘asphalt cowboys’, with turned up engines, are actually some of the safer drivers, since they often take more pride in their abilities, and in their rigs. There are exceptions of course.
When being overtaken, I routinely let up, just a little, to allow a truck by that was taking too long to pass me, whether on a hill, or flatland.
Interesting about comparable power.
I kept waiting and waiting for the guy in the right lane to let up a bit so the passing truck could complete his pass. Meanwhile, the line of cars behind us grew and grew and grew. BOTH those drivers were really discourteous to the long line of cars.