I can identify with this. I've noticed when I am tired, that my highway driving slows down.
It feels fast enough to suit me, but the speedometer indicates I'm doing the legal speed limit, or slightly less -- a radical departure from my normal behavior.
For one who has her foot in the carburator 90% of the driving time, slowing down my driving is not what I like to do. It's probably a good thing the truck is only four cylinders...