Optimal highway speed for fuel efficiency?

The manufacturers design to the test cycles so that they can hit the fuel economy standards. The US06 "highway cycle" spends a significant amount of time at 65mph, some time at 70mph, and only a very brief amount of time in the 70-80mph range. Peak fuel economy is going to be in that 65-70mph range, because that's what the engineers focused on

 

Stay in high gear an in middle of operating range where engine makes it best torque at its lowest rpm. For me that is anywhere from 60-70 mph.

 

Agreed. This engine is most efficient at the mid operating range. I now get the logic that the automatics have when they down shift at the slightest puff of headwind. I no longer go straight to 6th if driving at city speeds and I do get better mpg.
Maybe at less load, the engine is in atkinson mode vs. Lugging and loaded, in otto cycle.
Just a guess.

 

85mph in my manual would land me less than 15mpg. I got 15.9mpg on a road trip last year goin 70-75mph. Yes manual gear ratio is different and not as efficient.