Hopefully Mike Mas will join in on this one. Have you been able to figure out what's the most efficient mode overall? Normal, Eco or Volting (using Charge and EV sessions). Or has this already been flogged to death?
When we got a new ICE car, searching efficient mode has been flogged to death. Most of the people know the answer is "Slow start and steady speed". As the modern tech makes it possible to use electrons in real life, the "How to use it" becomes an significant issue, since we do not know the details of technical background in general. All is new to us and it changes very fast.
PHEV is not the exception of these new trend of fast changing tech. Hybrid engine and battery tech itself is not same compare to 10 years ago. Current Hybrid fuel efficiency is double than 10 years ago, and 2 miles for 1kwh(Mits) battery became 5 miles per 1kwh now (Tesla).
So, individual experience could be a poison to the others since we do not know the base of such individual experience came from. This is the reason why we can see so many different opinions against one simple question.
Based on the basic tech of PHEV in general, there are 2 different approaching ways. One is conventional and the other is modern tech. Conventional aspect is based on the Physics Law of Moving Object. In this way, we can talk about fuel efficiency under the Slow Start and steady speed. Eco mode is the nice sounding express of slow start and steady speed. Modern tech aspect is based on techs applied to the car by engineers capability. This applied to Hybrid Engine in general and Save/Charged mode button. This selection is for effective use of the car, including fuel efficiency. Once we mix 2 different function in one bowl, there is no answer exist.
I want you not to be confused with other individual experiences regarding "Efficiency". Modern equipment is made by modern technology, and modern technology went too far away from us to understand. Nevertheless, if you want to know your car for proper use and maintenance, get familiar with Hybrid history and battery tech. Simple question can not get you the right answer for deep and wide range of modern tech applied product.
I am very much sorry for useless comments, but I found so much articles with no understanding of PHEV. You can get any other useful comments from this forum about good solutions or how to solve issues, but not proper comments about efficiency.
These are the subjects on PHEV
What is Hybrid and how we save fuel by Hybrid
What is the Series in Hybrid
What is the difference between Hybrid and Plug-In Hybrid
What is Series and Parallel in PHEV
What is the meaning of EV distance in data sheet and number shown in display.
How to charge battery, what controls the battery
Good luck to you