Sacramento-Las Vegas road trip, 1200 miles r/t

I just got home from a 1130 mile road trip, Sacramento to Las Vegas, and have some fuel economy stats to share. The vehicle was a 2019 Prius Prime. Total payload weight including myself, luggage, and a spare tire and wheel, would have been about 440 pounds. The tires were the OEM Toyo's, inflated to 36 psi all around. The car is equipped with the OEM roof crossbars, but I didn't use my Thule cargo pod this trip, so no appreciable aerodynamic drag from external add-ons. The route taken involved about 11,500 feet of elevation gain and descent in each direction.
Leg 1: Sacramento to Bakersfield on CA-99, 270 miles. This freeway is about as level as a billiard table, with only about 250 feet of total elevation gain, mostly from climbing freeway overcrossings every few miles. I drove this leg at an average of 55 mph, and the car got 71.4 mpg.
Leg 2: Bakersfield to Las Vegas via CA-58 to Barstow, then I-15 the rest of the way. 291 miles. This leg crosses several mountain ranges and almost all of the 11,500 of ascent/descent happens here. I still drove at 55 mph, but as you would expect, fuel economy was 10.1% less, due to the terrain: only 64.2 mpg.
Leg 3 (return trip): Las Vegas to Fresno, CA, 405 miles. This was on the same route, but I was trying to see if the car would make it all the way home, from Vegas to Sacramento on 1 tank of gas, so I didn't stop in Bakersfield. 114 miles further along, the fuel gauge was almost down to the 1/8 mark, and I decided to not risk running out of gas, so I filled up at a station a little north of Fresno. This return trip, I drove between 65~70 mph, to see what effect higher speed has on the Prius Prime. The higher speed, combined with the 11,500 feet of elevation gain, yielded the worst fuel economy of the whole trip, at 52.7 mpg.
Leg 4: Fresno to Sacramento, 157 miles, average speed 65 mph. This leg is back on flat terrain, and the car got 58.3 mpg.
NOTES:
1. The fuel economy numbers are all from dividing the odometer mileage by the actual gasoline consumption recorded at each gas stop; this car's built-in MPG computer has always been too "optimistic", and states MPG numbers that are on average about 7% higher than actual.
2. I have a Bluetooth OBDII scanner installed, and had the Hybrid Assistant app running on my phone on all 4 legs, so that I could monitor powertrain performance in real time. This app shows very interesting numbers, like the actual power flow, in amperes or kilowatts, flowing into and out of the traction battery, the temperatures of the two motor-generators, both stages of the Inverter, the traction battery, and how much power the ICE is generating when it's running. By watching the energy flow and the duty cycle of the ICE at various speeds, it becomes painfully obvious that the Prius Prime's powertrain is far more efficient at 55 mph than it is at 70, and in fact, I read somewhere that that the entire powertrain (the ICE, the two motor generators, the PSD, and the traction battery) are most efficient at about 45 mph.

Lessons learned and problems encountered:
1. The Hybrid Assistant app shows enormously more information than the car's instrument panel, and most significantly, actual temperatures of critical components like the inverter, the motor-generators, and the ICE cylinder head temp are displayed. On flat terrain at 55 mph, in EV mode, the power draw from the traction battery to MG2 is about 8 kilowatts. This can double while climbing a 4% grade.
2. The typical duty cycle in Hybrid mode looks like this at 55 mph on a flat road: the traction battery is powering MG2 in EV mode for about 30% of each hour of driving, the other 70% of the time, the ICE is running, propelling the car and spinning MG1 to add charge back into the traction battery. At slower speeds on city streets, the duty cycle is even more impressive: at 35 mph, the car is in EV mode nearly 70% of the time, and the ICE only runs 30% of the time.
3. But at higher speeds, the duty cycle more or less disappears: at 70 mph, the ICE runs continuously, and only shuts down when the car is descending a downgrade steep enough that power demand drops to below 1 kilowatt. This is made very clear by the fuel economy numbers: the car got an astounding 71,4 mpg when I was driving it gently, at 55mph, and this dropped to only 52.7mpg when I was pushing the car hard on the return trip at 70 mph.
4. The Prius Prime hates steep terrain. The overall powertrain efficiency in mountains is far less than on the flat. There are a couple of very long (10 miles+) ascents on CA-58 and I-15 that average 4%. When I tried to climb these at 55 mph, the cooling system was apparently struggling a little to keep the ICE's cylinder head temperature within safe limits. Hybrid Assistant reported the ICE temperature at 195°F almost all the time on flat terrain, but on those 4% grades, it rose past 205°F and the indicator went red, so I slowed to 40 and did as much of the climb in EV mode as possible, until the traction battery flat-lined. The MG2 temperature also climbed to it's peak level for the trip, about 175°F.
5. If you use Hybrid Assistant, you will see that the power scale for the ICE goes from 0 to 65 kilowatts (87 hp). On very steep terrain, above 5% grade, I would not attempt to climb a hill like this at any speed that requires more than 50 kilowatts from the ICE for more than maybe 1 mile. 40 kW seemed to be the limit for power generation that the ICE cooling system can handle continuously before the head temperature begins to rise above 195°F. On flat terrain, at 65 mph, the ICE runs at about 25 kW, at 55 mph, about 18 kW, and at 45 mph, at 12kW. High speeds on steep hills also stresses MG2 and the inverter, and you can see this as the temperatures rise on long steep hills. NONE of this information is conveyed on the car's instruments, and the only warning that you will ever get of a problem is the Ckeck-Engine light (MIL) will come on, or the car might quit running completely. You don't ever want to overheat any of these components; the motor generators and the Inverter are all hellishly expensive to replace if they fail, like $1200 for the MG2 stator, and $1500 to replace or rebuild the inverter.

So there you have it: real-world fuel economy numbers, and the hype about the Prius Prime is genuine: it really can get 70+mpg, at least on flat terrain, when driven gently and you are not in a hurry to get to your destination.

 

Yes, the temperature numbers I quoted are all from Hybrid Assistant - but the source of the data is the car's ECU, and the various sensors scattered throughout the powertrain. Neither the OBDII scanner nor Hybrid Assistant are manipulating the data, they are only displaying the data being provided to the CANBUS by the ECU computer(s). At least that's what the author of the software claims. Thanks for the reply here - if the ECU on the Gen3 Prius doesn't cause a Fault condition until 221°F, then I need not have been worried when the temp in my car hit 205°F briefly on a steep climb.