I'm sure that each car has a "sweet spot" where they mileage is maximized at a certain speed, but to argue that it's 55 MPH for every car is ridiculous. Maybe those sweet spots should be identified for each vehicle.
Generally around the speed where the car is first able to run in "high gear".
I discovered the "sweet spot" in my 1997 Saturn SL1 while on a trip up north. (I won't say what provinces).
Did a hundred for one complete gas tank... got 45+MPG!
At 75 MPH down here I get 39 to 40 MPG.
At 65 MPH I get 41 to 42 MPG.
I don't know what it gets at 55 MPH.
The thing has 415,000+ miles on it now, still going strong without a squeek or a rattle.
Exactly. In aviation it's referred to as "stoichiometric" and thats why some cars labor and get worse mileage at lower speeds and some a higher speeds.