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Fuel Economy Indicator Seems Off

Discussion in 'Gen 2 Prius Fuel Economy' started by Liberty B, Sep 10, 2021.

  1. Liberty B

    Liberty B New Member

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    Hey there!

    I'm brand new to Priuses and know very little about them (forgive me!! I do know quite a bit about non-hybrid cars!)

    I bought an '08 recently, and my fuel economy is supposedly averaging around 46 mpg. OG battery.

    NOW- by how fast my fuel tank appears to be dropping, (and I haven't yet calculated this myself- Ive only put gas in the tank twice since owning it,) it certainly does not seem to be getting 46 mpg.

    Can someone please tell me if the fuel "gauge" could be off, or the (whatever computes your mpg) or both, and why??

    I'm honestly not versed in cars that are this electronic/computer based at all, but am a huge fan of efficiency.
    For reference, my other car is a (manual) '02 VW Golf TDI (diesel; it also runs on biodiesel)...which averages around 40 mpg.

    Thanks!!
     
    #1 Liberty B, Sep 10, 2021
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  2. bisco

    bisco cookie crumbler

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    three issues here:

    1) the dash average is 3-5mpg optomistic, and the speed reads 1-2mph higher than actual

    2) the gas tank has a bladder, and does not fill the same each time, depending on ambient temp

    3) the fuel gauge is not linear, unfortunately

    4) welcome to the world of prius quirks! :D
     
    #2 bisco, Sep 11, 2021
    Last edited: Sep 11, 2021
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  3. Mendel Leisk

    Mendel Leisk Senior Member

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    Slight hyperbole there. :eek:
     
    #3 Mendel Leisk, Sep 11, 2021
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  4. bisco

    bisco cookie crumbler

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    thanks mendel!:p
     
    #4 bisco, Sep 11, 2021
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  5. fuzzy1

    fuzzy1 Senior Member

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    My best guess: that was a typo of "1-2mph higher".
     
    #5 fuzzy1, Sep 11, 2021
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  6. dolj

    dolj Senior Member

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    To mitigate this, you need to keep track of your distance and fuel usage so you can accurately calculate your MPG. You then need to smooth the average using a 5 tank rolling average.
     
    #6 dolj, Sep 11, 2021
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  7. jerrymildred

    jerrymildred Senior Member

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    That fuel tank bladder in the Gen 2 is a strong candidate for the most aggravating Prius annoyance of all time, especially in cold weather.
     
    #7 jerrymildred, Sep 12, 2021
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  8. Liberty B

    Liberty B New Member

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    "5 tank rolling average?"

    Thanks!
     
    #8 Liberty B, Sep 12, 2021
  9. dolj

    dolj Senior Member

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    Yes.

    You combine the data for the last 5 tanks and average that. Each time you fill, the previous 5th tank's data is dropped and the new tank's data is added, hence as time goes on the average is 'rolling'.
     
    #9 dolj, Sep 13, 2021
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  10. fuzzy1

    fuzzy1 Senior Member

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    Hardly anyone goes to that much effort. Most other people would just average however many consecutive tanks as they have recorded.
     
    #10 fuzzy1, Sep 13, 2021
  11. dolj

    dolj Senior Member

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    The effort is almost nil if using a spreadsheet, once the initial formula has been entered.

    Obviously, one does what is the easiest to do with the resources available.
     
    #11 dolj, Sep 13, 2021
  12. fuzzy1

    fuzzy1 Senior Member

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    "Almost nil" is still above the effort level of most drivers. Most won't use computer spreadsheets, or even a paper logbook. Mobile phone apps seem to be the most common fuel economy tracking tool mentioned here on PriusChat.
     
    #12 fuzzy1, Sep 13, 2021
  13. dolj

    dolj Senior Member

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    Hmmph, I thought I covered that by saying, 'Obviously, one does what is the easiest to do with the resources available.'
     
    #13 dolj, Sep 13, 2021
  14. Another

    Another Senior Member

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    #14 Another, Sep 13, 2021
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  15. pgaster

    pgaster New Member

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    This!
    I can fill my 2006 and drive it to work for 2 days before the gauge starts to drop on the 3rd day.
    I have roughly a 38 mile commute one way, so that is a bit over 150 miles before it shows movement.
    To be fair most vehicles I have owned seem to behave the same way but not to this extreme. Most of the time for other cars it would be maybe 30 to 50 miles before the gauge moved, not 150.
    My bet is the manufacturers do this to make everyone have a good first impression of the fuel economy.
     
    #15 pgaster, Sep 16, 2021
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