I am pretty confident at this point that the Prius Prime PEHV has substantially higher fuel economy than the Prius HV. I had a 110-mile round trip in the HV mode with eco mode yesterday. It was mostly highway driving, with typical speeds in the 50 - 65 mph range, going through hilly areas at times, temperatures in the 60 - 80 °F range. Part of it was stop-and-go or city. I was monitoring the EV battery, and the usage of the EV battery is aggressive -- typically charging and discharging to 5 - 10% of the capacity. I verified that the initial and final EV battery level were the same. I averaged 75+ mpg. I doubt you could get this gas mileage with the Prius HV. I think the EV battery is substantially helping optimize the HV driving, as long as you have at least about 10% EV-battery level left.