Tank MPG vs calculated MPG randomly off?

Discussion in 'Prime Fuel Economy & EV Range' started by pakitt, Sep 17, 2022.

  1. pakitt

    pakitt Senior Member

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    I use Trip A to keep track of how many miles I travel between fuel ups (gas). Trip B is dedicated to track single trips stats.
    Usually the average MPG reported by my Prime is off by about 4%. But I am now noticing it is sometimes off by about 10% and a few times even 20% (optimistic).
    At first I thought the gas station I last went has pumps that are willingly or unwillingly "off". But this has happened at different gas stations at different times since I own the car (i.e., since Feb. 2021), so I don't think it is a particular place saying it sold me X gallons when instead they were less than that.
    Does anybody also notice wildly off MPG estimates?
    I understand that it might not be easy for Toyota to provide accurate MPG estimates when mixing EV and gas trips together, but swings from 4% to 20% seems a bit too much.
    When I had a gas only Prius the MPG was around 4% optimistic, ranging from 2% to 10% - but never so much as I am observing in the Prime.
     
  2. bisco

    bisco cookie crumbler

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    i don't think they have a problem due to hv/ev. after all, it's just a function of miles traveled and gallons used.
    most toyotas are optimistic by a fe percent as you said. same with the speedo. but 10-20% does seem like something is off.
    it's more likely a tank filler neck valve/evap vapor recovery issue than a software glitch, especially if it is intermittent
     
  3. Salamander_King

    Salamander_King Senior Member

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    Yep, that's completely normal on the PP. The mpg display on the dash is always wildly off from the calculated (full-tank method) mpg. It is much worse on PP than any other cars I have kept the records. But after many data points, on average the dash MPG display is off by 6-8% on the optimistic side. This was true on all three PPs I owned. The average difference (dash MPG) : (calculated MPG) were 2017:+6.02%, 2020:+7.96%, and 2021:+6.44%.

    Here is the data on my 2021 PP so far. The difference between the full-tank method calculated and MPG on the dash is different from -10.30% to +17.88%. The discrepancy is generally wider if I drive many miles on EV in between the tank fill-ups, but that's not always the case. The wide discrepancy was also observed in my previous 2017 PP with over 50 fill-ups. (the second table showing just two columns). It was from -1.09% to +19.96%.

    Compare that to the same data analysis done on my 2015 Gen3. As your Gen4, the distribution was much tighter. The bottom table is a portion of data from Gen3. As you can see, the discrepancy was between -1% to +4.5%.

    upload_2022-9-18_21-40-7.png

    upload_2022-9-18_21-50-36.png

    upload_2022-9-18_21-56-20.png
     
    #3 Salamander_King, Sep 18, 2022
    Last edited: Sep 19, 2022
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  4. pakitt

    pakitt Senior Member

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    Thanks for sharing your data @Salamander_King ! By the looks of it, it seems pretty normal and also that either the software or some other physical process is at play here. The variations reflect my PP behavior and I cannot imagine brand name gas station stealing up to 20% of gas. I can see them shaving off 1-2% (or more?) per each tank of gas, but 10-20% would be a scandal.
    Looking forward to that 700mi range EV Prius that Toyota is still dreaming to make with solid state batteries, and then this will be a problem of the past.
     
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