I used to work for Bob Crawford, a retired truck fleet owner in Omaha NE. He kept one truck after he retired and I drove it for him. It was one of the best looking trucks on the road and had a Cat engine with so much power, it was almost like driving a car.
Occasionally Bob would take a trip with me from Omaha to the west coast. On one of those trips, we were going through Wyoming on a 100 mile stretch of I-80 called "Elk Mountain", which was well above 7,000 feet. The weather was clear, the road had been recently re-paved, and the big Cat engine was really cranking hard.
That engine would get really quiet when it was running at max load. About the only thing you could hear was the muffled whine of the turbo through the twin chrome stacks. We had been cruising along like this for a while when Bob turned to me and said, "Dave, why are we going so slow?" I said, "Bob, look at the speedometer."
The speedometer needle on that 60' long, 13' 6" high, 80,000 lb truck was sitting right on 80mph.