which mode to use in prius prime

Mode means more than one thing. The mode you asked about is all about the sensitivity of the accelerator pedal. Eco gives a slower start for a given pedal push, and may add some range. Normal gives a brisker start for the same push, and Pwr has the most giddy-up for the same pedal push. We usually use Normal, but switch to Pwr on the freeway when we might need to squirt into an opening in traffic. Try them all and see which suits your driving.

The other totally separate Mode setting is for EV, EV Auto, or HV. EV does everything using the battery first, then switching to the engine when the battery is depleted. You can leave it this way all the time. It's smart. EV Auto uses the engine on a hard pull even if battery power is available, and may give slightly better mileage if the hills are steep. HV is standard hybrid which is a little battery and lots of engine use. I've never found a reason to switch to this, and the EV mode does it automatically for me.

 

Congrats on the new ride. You can get good economy in any of the three modes depending on how you drive. As mentioned, it remaps the throttle sensitivity. Here's a chart from previous generations. Gen 4 is similar if not identical


I find that, if I'm driving slowly (45 mph or less) in EV, Eco gives me very fine control over the m/kWh I'm getting by allowing me to keep a steadier speed. I would suppose that at highway speed in HV mode, you'd have a higher throttle opening and maybe it might be better in Normal or PWR since they have shallower slopes at higher accelerator openings. But that's a guess since the graph doesn't correlate speed with throttle position.

 

I devoutly wish Toyota called the two types of "mode" something different.

Pedal Sensitivity, maybe?

 

But "pedal sensitivity" isn't all they affect.