How was it debunked that 55 saves gas? I'm interested in this, as I sometimes like to drive in such a manner as to economize, and according to my instant and av

How was it debunked that 55 saves gas? I'm interested in this, as I sometimes like to drive in such a manner as to economize, and according to my instant and average mpg readouts cruising at or below 60mph consumes far less gas than cruising at or above 70mph.