Concur:
- 55-65 is best. I consistently get >40mpg at 60mph above 40deg, day in and day out. Over 70mph or below 40deg drops me like a rock.
- "eco" just means you are getting better than 30mpg at that instant in time...most often by being on 3 cylinders. So even if its lit up 100% of the time you can still be getting only 30mpg, as you have seen.

The engine cannot supply the necessary power with 3 cylinders above 82mph- what you saw was completely normal.

FWIW, the HAH still rides very nicely with 38psi in the tires- try that and see if it bumps mpg up. I run at 40-42, but it starts to feel a little sporty then.