Depends on the car really. I have a 6 speed manual in which 6th is overdrive. I think the best mileage I ever got with my car was averaging about *redacted* MPH to Vegas.
Wasn't 55. That gear doesn't even become useful until about 70 MPH
Not trying to start a fight but people can test their cars to see what speed nets them the best mileage.
It's actually about 45 mph, I think. Wind resistance isn't linear, and increases fast above that speed. I see the best economy about 45, if you can find a road where you aren't stopping all the time.
Caption this Meme 