The 4 modes and the quest to the best MPG

Six months have gone by, and I am starting to get a hang out of my new 2021 Prime. This is my 3rd Prius. The first one to own on US soil, the other two being owned in Germany.

I have driven 4600k mi so far and long enough trips to see how she behaves when the ICE gets used. I also experimented a bit on some drives to see what happens with EV Auto and HV+EV modes.

I also made an excel table to see how much 30 or so miles in EV mode affect the cost of a trip vs. using HV only.
My assumptions are 31.8mi of EV range on a full charge (ideal); local gas costs of $3.35/gal average; electricity cost of $0.146/kWh; EPA average of 54 MPG (ideal).
The table tells me that I save 15%-50% depending on trip lengths of less than 100 miles. After that, the savings go down to 3% with trips of up to 500mi.
Of course, the fewer EV miles are driven (because of speed, terrain, temps, etc.), the lower the overall trip cost benefits.

I am still wondering, though, if it is possible to improve the MPG on longer trips (more than the EV range) by using the different modes. In this regard:

1. I am still very perplexed by EV Auto: EV mode on the Prime is "strict," meaning that you can press the gas pedal as much as you want, and the ICE will not fire up (which is actually nice). So by default, the Prime starts in EV mode to drain the battery, and then switches to HV mode no matter how I drive. EV Auto, with my driving patterns, behaves exactly the same. I also never really experienced a behavior for EV Auto like the one explained in the user manual (i.e., to give you "an extra oomph"). At highway speeds up to 85mph the Prime merrily uses the SOC, as if in pure EV mode, instead of switching to HV mode, which I would think to be more efficient. EV Auto seems pretty useless to me. I am reading on other posts/threads that starting up the Prime in HV mode, getting the ICE warmed up, and then switching to EV Auto will change its behavior and be more helpful. Is that a fact? And in general, should EV Auto mix HV and EV mode to work like an "enhanced" HV mode or an "extended" EV mode? What is EV Auto for at all? Is it a gimmick, like EV Mode in the Gen 3 and Gen 4 is? (The EV-only range and management on the Gen3/4 is essentially useless; you can simply accelerate lightly on a decent SOC and achieve the same result as EV mode. If you accelerate too much, EV mode will be disabled anyway. I used EV mode on the Gen3/4 only to move the car within a parking lot or a driveway, to avoid warming the ICE up.)
2. CHG Mode: is it known how much extra fuel is getting burned to charge the battery to x%? Is there a situation in which burning fuel to charge the battery is more helpful to improve overall MPG? How does it compare to simply use up all the EV range and then go into HV to achieve the best MPG?
3. Based on my excel table, and this is more of a statement than a question, it is clear that using up all the EV range before coming back home to charge it up is the best way to save fuel vs. HV mode only. My impression is that the best MPG is achieved if you can get to 0% SOC during a trip before the next recharge. Those 30 miles of EV range bring a tangible benefit on overall gas costs for trips up to 100mi. The question is, using CHG mode during a trip can further improve that? Can EV Auto improve that? Otherwise said: in a trip longer than the EV range, how can one achieve the best MPG by managing the SOC and "mode" depending on the type and length of the trip? What advice do you have based on your personal experience? In "vanilla" Gen 3/4 Prii, one had to hypermile. What do you do with a Prime? Hyper-switching modes? Can one simply "drive it" and get the best MPG, or does one need to fuss with switching modes all the time depending on where and how you drive?

PS: If you are interested in having the excel table to calculate how much x miles of EV range saves you on a trip of overall y miles, please let me know.

 

I tried this morning to start the car in HV mode, get the engine to warm up, and when I saw that it was done I switched to EV Auto. I tried accelerating a bit more aggressively up to 55mph (the max I could do on my route) and nothing happened. Merrily going on in EV, no a blip from the ICE.
I think with DRCC on and EV Auto selected the ICE will fire up because the DRCC has the bad tendency to accelerate very aggressively to keep up the speed. Not very environmentally (or MPG) friendly, IMO. I remember on the Gen3 and Gen4 I owned, that if you put the car in Eco mode (or power) it will respect the setting in accelerating while trying to keep the speed in DRCC.

Long story short - for me EV Auto is useless and it is a pity to drive a 21st century drivetrain, with software from 1990. Like the the entertainment huge screen in the middle of the console with pretty basic graphics and tiny space for Car Play - what a waste. I rented a Gen4 basic last month, and the screen was nice and wide, better than this useless "thing" in the prime. Toyota marketing...

Yes, I agree, goal of the game is to come back home always with SOC to zero (assuming one drives more than the 30 or so miles EV Range). And in the case you describe, with my table, the advantage to drive EV and then HV for the rest of the trip, goes down significantly (6-17% savings vs driving HV only, instead of 15-50% in my case, for trips shorter than 100 mi)

 

Yesterday I did a round trip to Denver. Started with EV mode on a full charge before entering the highway. The drive to the ramp is about 7mi of very hilly terrain. As I started to go down the last hill, I activated HV mode to make sure the ICE was warmed up before I took the highway. I don't know how "smart" the Prime is in warming up the ICE from EV mode to HV mode, while driving at 60+ mph...

When I arrived in Denver, I turned on EV mode for the city driving section of the trip.
On my way back I did the same, EV mode in the city, HV on the highway.
When I started going downhill on the highway before getting to my off ramp heading home, I turned EV Auto. To my surprise the ICE turned on and pushed the car at speeds of about 50mph (I kept the accelerator pedal in the same position as I made the switch between the modes) and my battery was over 50%.
So, meh, still confused about EV Auto mode...