Like the ordinary Prius, PiP was designed so you can "just drive it" and do quite well on efficiency. And like the ordinary Prius, a little extra attention to technique can bring big efficiency wins. PiP's extra trick is that judicious use of the HV/EV button seems to result in even higher MPG. My question to PiP experts is: can we come up with some simple rules of thumb for when HV is clearly more efficient than EV, and vice-versa? So far consensus is that on hills and freeways HV mode is better because using both ICE and battery is more efficient than using either alone would be. Q: Can I know from the HSI alone when a hill is steep enough to switch from EV to HV? Like if the EV demand bar is over 50% or something simple like that? What about on a flat with no wind. At what MPH would manually switching to HV be clearly better than EV alone? I guess in order to know that we'd need to compare (rate of gas consumption + net rate of kWh draw) in HV mode with (kWh draw) in EV mode for various situations. And we have some rough equivalency of gallons to kWh (maybe 33.4 kWh /gallon). I'd make a bosh of the calculations, but I bet some of you scangauge jockeys have been doing this. So give us your pithy wisdom, please!