Thanks for the backup.

Well, finally finished my tank, and in this totally non-scientific test, I found out that the mileage improvement is quite low for this type of vehicle, if any.

I've asked questions around, and the best explanation I've got is that a normal ICE benefits the most from premium gas while accelerating....something that in the HCH is provided in large part by the electric motor.

So, that is good news... one can get optimum mileage with the cheapest gas grade.