Ok, before we all are going to “kill each other” on how well we remembered the stuff we had to learn during physics class, let us go back to a practical example to figure out why you should use EV mode in the city and ICE on the highway. That is if you have the PHEV.
As someone rightfully stated the power an ICE can produce is not constant over the total range of RPM available to propel the car. This has been THE engineering challenge forever since ICE were used in vehicles. To optimize the solution engineers worked forever on designing transmissions and the fuel burning process. Electromotors have the ability to pretty much use the maximum power over the complete RPM range. That is why there is no need for a transmission in an EV.
Engineers have studied and experimented at length what will create overal average the best fuel consumption for a car without sacrificing too much performance. The result remains a sub-optimal energy consumption.
I can’t say my driving is typical, but I have driven 2 cars that are about the same size and shape (Niro EV and the BMW X1 2L Turbo) both having motors with roughly same power (150kW) and I drove them under the same circumstances (driving style, terrain). The long term average on my EV is 16.1 kWh/100km, my X1 consumed on average 12l/100km. I know it is metric, but for comparison this makes it a bit easier. So how much energy is in 12l of gasoline? According to this answer by a Professor
12l gasoline contains 108 kWh of energy! If I would assume that the BMW engine is highly inefficient (BMW would disagree!) at 20%, then there is almost 22kWh used per 100km to obtain the same driving. Which shows the inefficiency of the transmission, but also the lack of the ICE being able to regenerate forces to slow down the car back into the energy form it has on board (gasoline).
So long story short for the PHEV drivers: use EV mode in the city and ICE on a longer highway stretch as soon as you arrive at cruising speed.
Is the car able to detect this and automate this for you? So you don’t need any of the knowledge while driving and can listen to the music? Only when it has a location based system with data on the type of road you are on in combination with real-time traffic conditions, so it can reliable predict the driving style (e.g. GPS based navigation system you can find in the Niro). The computer making the decisions which engine to use will need this info in combinatinon with speed and the throttle position. Since a nav system in a car is still being sold as a way to find your way and still seen as a luxury for which you can charge additional money, it is not yet used for this purpose. Maybe some day it will become a competitive advantage for a car manufacturer to automate the optimal use of motors in a PHEV. I am sure the engineers have this already worked out, but the product managers and marketeers are not yet in. Time will tell!