I have never done any engineered study but just based on my normal routine on highway to work observed over a period of time.

I owned an Axia 1L and been using it for 5 years. It is an auto. Could not remember the stated spec but after refueling the car on the way to work. Reset the car fc meter to zero I can get about 20km/L. Tried that on numerous occasion. Roughly the same results. I think the spec called for 21.x km/L. Speed around 80km/h or 100km/h I see no difference. On way back the traffic was more heavy and the average dropped to about 15~16km/L. It would improved back on the morning to office.

Now I owned a MyVi 1.5L auto. Similar test I can get around 21km/L but I thing I observed was that the MyVi improved its fc average much faster than Axia. My observation was that on highway the MyVi can average about 18km/L and Axia about 15km/L on routine to and fro to office over easy and and heavy traffic. In city driving, the MyVi gets around 15~16km/L and Axia was around 12~13km/L.

All these were based on normal routine route on highway and also in city.