I’ve always assumed it’s something in the engine. But I also wonder if it’s some sort of friction based math? I have heard most cars are most fuel efficient at 55mph or so. My town has a large hill which my car can “coast” down with no gas required. However, like you, it stops increasing its speed around 55mph. It doesn’t seem to go any faster without me stepping on the gas. I don’t notice anything different about the car. It just seems to stop accelerating.

And if I hit the gas, it seems to re-settle back to 55mph.

I sometimes wonder if fuel efficiency being at 55mph is tied to friction of tires on asphalt. And maybe even on a steep hill, typical cars just cant overcome the kinetic friction to continue speeding up. Obviously the steepness of the hill matters, but roads have pretty stringent rules about grades. So we have an upper bound of what’s “reasonable” based on whatever the steepest allowable grade is.

I’ve also had this question. I’d love to know the answer!