My experience with my former Prius C and some limited time in a rental GenIII Prius, the EV icon appeared to correlate exactly with the ICE not spinning. I never saw it above 46 MPH on the C, which agrees with a few websites that explain the Hybrid Synergy Drive and the operational parameters being determined by maximum speeds of any of the three motors. I wonder if the EV icon on the Gen 4 lights up when the engine is spinning, but the fuel injection is off, and the system is powered by battery. I observed this mode on the C, but the EV icon stayed dark in that mode, even though in that mode it effectively it is an EV vehicle.
i think you really need it to understand what's really happening. toyota's gauges are all fraudulent. hopefully someone who has one can do some testing.
