Some Real-world range numbers in a 2019 Prius Prime

Discussion in 'Prime Fuel Economy & EV Range' started by Laura-Ann, Sep 14, 2021.

  1. Laura-Ann

    Laura-Ann Junior Member

    Jul 11, 2019
    Sacramento, CA
    2019 Prius Prime
    Prime Plus
    I have a 2019 Prius Prime (whatever the base trim is, mine doesn't have the bigger display screen that some of them do). I've driven it 17,561 miles as of this morning. I've been tracking fuel purchases since day 1, and since last July, I've been noting the discrepancy between the odometer mileage/real mileage, and the car's reported MPG/actual MPG. All of this mileage is on "California Blend" gas, which is 10% ethanol.
    1. The car still has it's OEM tires. The odometer consistently reads too high by 1.22% on average. This means that for every 1,000 miles that the odometer says you drove, if you were to measure the actual distance on a map, or had a tracklog of the trip recorded by an independent GPS, you would have actually driven only 988 miles.
    2. The speedometer reads about 4% too high. If it says 65 mph, you are actually driving 62. I've checked this against both a GPS, and those road-side "Slow Down!" radar signs you see at construction sites and school zones.
    3. The car's MPG display is too optimistic: over the last 3 gasoline purchases, all made at the conclusion of long road trips, the number reported by the car is consistently too high by about 5.1% (this is computed by factoring the odometer error and the MPG error together). If the MPG display says you are getting 60mpg on the current tank of gas, you are really getting 57. With this in mind, I would suggest caution regarding the car's "Estimated Remaining Range". If it says you have 100 miles remaining range, you might have no more than 95, or perhaps even less, you can never be completely certain down to the ounce about how much gas you have left.

    I just got home from a 490 mile road trip to Kings Canyon National Park. About 300 miles of the distance was on CA-99, on flat terrain, the other 190 in the mountains, with a total elevation gain/loss of 23,800'/-13,600' (round trip). This trip used 7.146 gallons of gas. I left with the battery full, showing 37 miles EV range, and arrived back home with 0.5 miles remaining, so about 7.5 kw-hr worth of the energy used was from the initial battery charge, and the remainder was gasoline at an adjusted 68.7 mpg. A couple of factors to note:
    1. The car had a Thule cargo pod on the roof, which would normally cost some fuel economy, yet I still ended up back home with an astounding 68.7 mpg for the trip.
    2. On the return trip, from Cherry Gap, the highest point on the road at 6840 ft, to the bottom of the descent at the base of the mountains at 500 ft, the distance is 32 miles. That's an average grade of 3.75%. I arrived at Cherry Gap with the traction battery empty, and by the time I got to the bottom of the hill about 40 minutes later, the battery was fully charged to 37.5 miles range, just from the regen braking. I set the cruise control to 40mph and never touched the brake pedal the whole way down. On the steeper sections, where the grade was in the 5~7% range, I just used "B" mode, and the car had no problem keeping to the set speed, and since I did the whole descent in "EV" mode, and the traction battery didn't get full until right at the bottom, the car never resorted to spinning the ICE for compression braking. The Prius Prime is an amazing piece of tech. I've never been able to make a descent like this in any other vehicle without needing to at least occasionally use wheel brakes, or depend on compression braking. So I got to the bottom of that hill with 37 miles of "free" driving in the bank!
    3. Another noteworthy thing about this descent is that the traction battery was able to absorb 8 kw-hours of regen braking energy in only 40 minutes, yet it takes 130 minutes (3.5 times longer) to charge on an L2 charger. I wonder why that is? If the Prius Prime traction battery can recharge on regen braking at a rate of 12 kwatts (8 kw-hr/0.67 hours), why do they put a puny 3.3 kw charger in it? I am guessing this to extend the life of the battery by charging it sloooowwwwlllyyyyyy.
    4. Speed: I never drove faster than 55 mph on this trip, and the average was about 40 (120 miles of the distance was on twisty mountain roads). I have occasionally been in a hurry to get somewhere, and I can tell you that the Prius Prime's fuel economy suffers a considerable hit at speeds above 55. MPG at 60 is about 58 mpg, at 65mph it's 54 mpg, and at 70mph, it's about 50 mpg. This difference in fuel economy is remarkable: the car gets 35% better fuel economy at 50mph than it gets at 70mph.
    PiPLosAngeles, Gokhan and dig4dirt like this.
  2. Gokhan

    Gokhan Senior Member

    Oct 5, 2008
    Paramount CA
    2021 Prius Prime
    My observation on the speedometer is that it shows 2 mph higher than the actual speed, regardless of the actual speed (perhaps except at very low speeds). So, it seems to be a 2-mph but not a percentage overestimation. However, I will admit that I never did a systematic study with GPS.

    The MFD MPG seems to overestimate by about 5 mpg or so. Moreover, in HV driving, EV battery is typically used up to 5% or so, which is about 1.7 miles of driving, which increases the observed mpg. So, if you drove about (17 miles) + (1.7 miles effectively from 5% EV-battery use) = 18.7 miles, you would see a 10% mpg bump due to EV battery use.

    Fast-charging decreases the service-life number of cycles of the EV battery substantially. That's why Toyota doesn't use it. Solid-state batteries are currently being developed to overcome this problem, but they are still far from reality.

    The biggest factor in fuel economy for HVs and/or EVs is the aerodynamic drag, which increases with the square of the speed of the car. Therefore, the slower you go, the higher your mpg will be. I obtained 100–110 mpg actual HV fuel economy (accounting for the EV-battery usage and MFD overestimation) in trips of 20-mile-long slow freeway traffic. The MFD would show about 120 mpg at the end of the trip.
    #2 Gokhan, Sep 14, 2021
    Last edited: Sep 14, 2021
  3. chickenhawk

    chickenhawk Member

    Sep 13, 2011
    2021 Prius Prime
    I noticed when driving in stop and go traffic I would make home on the charge but 2/3 stop and go and 1/3 regular speed I would spend 5-6 miles of gas. But then I'll be home 20mins later:mad: if in the stop and go all way.
  4. Mark McIntyre

    Mark McIntyre Member

    May 10, 2017
    Boulder CO
    2017 Prius Prime
    Prime Premium
    I just completed a 2 day, round trip from Boulder to Montrose CO and back. Started with a full tank and charge (~30 miles). Drove 625 miles at an average speed of 54 MPH (70-80 when traffic allowed) over multiple passes. By using charge mode during descents and in some flats I was able to have an EV ratio of 29% for the trip. No plug in charging at all other than the first 30 miles.

    Overall MPG with a lead foot and mountainous terrain: 56.5 mpg! I think that is pretty good. 2017 PP top level trim.
    Raytheeagle and Marine Ray like this.