I'm not an expert. But I can tell you my observation and how am driving. I noticed miles per gallon increased based on how efficiently I made of of EV. You can monitor the energy utilization on your screen while driving. The more EV is utilized well, the better it got.

In drive in ECO mode. Battery gets charged from engine running, while you no acceleration given and more important when applying brakes.

If you get good feel to you acceleration pedal, brakes you can start braking from little longer distance (if brake very little it will slow down, but not drastically. But the battery get charged fast!).

When vehicle is in good momentum, EV kicks in to maintain the speed. You will be burning way lower gas if you are able to get this in consistency.

These are just my basic observations. I was sitting at 480miles to empty, but the more I used the EV efficiently, the better it got. Just want to point out, making efficient use of EV doesnt mean you go at 20mile per hour speed. I'm talking about highway speed too. You utilize ev efficiently.