Small Update:
Returned from Ghaziabad->Agra->Ghaziabad trip of 550KMs. We were 2.2 onboard with some luggage i.e. ~200KGs extra weight. I tried different speeds to observe the speed FE relation. Complete trip was done with ECO mode without AC. Each speed was maintained for 90 KMs without brake. Only Yamuna Expressway allows you to experiment like this. Below is the relationship:
70KMPH-25KMPL,
80-23,
90-21,
100-19.
And yes, Eco mode can touch 140 kmph but struggles beyond it.