Thanks but you didn’t give references 
Did another search and have two more bookmarks:
- Coggan’s presentation “Cycling efficiency (efficiently)”
Summary:
- can vary significantly between individuals
- apparently determined by two major factors: a) biomechanical (saddle height, cadence), and b) biochemical (fiber type, SERCA activity)
- improves as a function of ordinary endurance training
- might be further increased with very prolonged and/or very intense training, and/or as a result of hypoxia or resistance training
- Blog post “Exercise Economy / Efficiency” at Training4Endurance.co.uk
Article discussing economy for the general endurance training audience (running, cycling, swimming) and includes plenty of references to studies. Somewhat similar conclusions as Coggan presentation.