The old ... '55 MPH' economy
The old ... '55 MPH' economy limit, wasn't an arbitrary figure pulled out of a hat, it was the result of physical reality. A vehicles drag _'squares'_ at just over 55 MPH.
There is a lot of potential MPG by driving at 50-55MPH and accelerating slowly up to speed.
Most people never hit the EPA figures because of the way they drive. They waste fuel and brakes and increase pollution by accelerating from one stop light to the next.
I drive like I have no brakes... saves a lot of money. And I never drive my big pick-up over 55MPH.
If you drive a manual, shift at around 1,800 to a max of 2,200 RPM, for best economy.