Announcement: please post your average speed and temperature when posting about fuel consumption

Discussion in 'Gen 4 Prius Fuel Economy' started by pakitt, Jul 12, 2016.

  1. pakitt

    pakitt Senior Member

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    If you post your average fuel consumption (MPG) for a specific trip or full tank without average speeds and ambient temperatures, the value itself has very little meaning.

    If you do 60 MPG at 40 mph average, it is quite different when you do it instead at 60 mph average.

    Posting simply "I did 80 mpg!!! WOW!!!" doesn't say much... ;)

    Thanks for your cooperation :)
     
    #1 pakitt, Jul 12, 2016
    Last edited: Jul 12, 2016
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  2. Jim in NC

    Jim in NC Active Member

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    Since I got the new Prius, I don't actually know where the average speed and temperature is located.

    On the old Prius it was on the tripmeter. Please let me know and I will post it. I always keep track of my tank miles on Trip A.
     
  3. pakitt

    pakitt Senior Member

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    The temperature is in the top right corner of the speedometer screen.
    The average speed (for each trip! :) ) is in one of the "i" information screens, 2 after the hybrid system bar screen. Press down to scroll to that screen.
     
  4. JohnF

    JohnF Active Member

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    Wayne Gerdes at cleanmpg.com did a speed vs mpg plot for the Gen 4 ECO 2:
    2016 Toyota Prius First Drive Review - First Drive Results | CleanMPG
    It's no surprise that mpg is very sensitive to speed. After all, loss of energy to form air drag goes as the square of the speed (according to the basic model), and other losses increase with speed as well.

    Wayne does not say, but I imagine this plot was done with the gas engine running. If so, especially at speeds in the 50-60mph range where the EV provides nearly enough oomph to maintain speed, one might do much better the more one can persuade it to run in EV.

    Incidentally, from Wayne's plot one would expect steady state 60mph to yield roughly 64mpg, and steady state 40mph to yield something over 80mpg.

    As for temperature, by now most people here are well aware that higher temps mean better fuel economy, at least up to the point where AC is needed and that loss factors in. Incidentally, I think that the temp shown on the upper right of the screen is the current temperature rather than some average. But I am not sure what an "average" temperature would mean anyway.

    As for average speed, it seems unlikely that 60mph average reached by driving some at 80mph and some at 40mph would give the same mpg result as driving 60mph the whole time.

    If someone gets a high mpg reading that is useful to me because it expands the performance envelope of the Prius. It's already clear to me that the speed must have been reasonable and the temperature favorable.
     
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  5. bisco

    bisco cookie crumbler

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    ouch! calling out the hyper milers!:p
     
  6. pakitt

    pakitt Senior Member

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    Yes and no. If somebody posts a 100mpg number driving 60mph with a warm engine, full battery on a 3km ride at 25C, it is quite different if it got the same number same speed on a 300km trip at 30C.

    What I am trying to say, is that MPG numbers out of context, don't tell much. And too often I have seen people posting here "great MPG" or "ugly MPG" and then when you start asking (annoyingly to be honest), all the parameters to understand how they got to that number, you realize that their MPG is actually pretty good in most cases, when "ugly" and ok/good when "fantastic" or even unrealistic (like 5km driven 80% of the time electric with more than 4 bars, 25C and no traffic on a straight flat road).

    It's like when I look at my Gen3 average fuel consumption of 5.34L/100km lifetime on Spritmonitor compared to other owners - I might think it is bad. Then when I think of where I drive, the average (chilly) temps we get here, the terrain, the average highway speeds, compared to a Gen3 owner in flat lands around Hannover posting 4L/100km, my average fuel consumption doesn't look too bad anymore... ;)
     
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  7. CR94

    CR94 Senior Member

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    Cruising or maximum speed would be more useful than an average.

    Another piece of data needed for claims to be meaningful would be altitude change for one-way trips. That is, if you scored 110 mpg on a short downhill jaunt, I'm not impressed.
     
  8. bwilson4web

    bwilson4web BMW i3 and Model 3

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    FYI:
    Actually the model is:
    • drag_hp = (mph * (Target_Coef_A + (mph * Target_Coef_B) + (mph * mph * Target_Coef_C) ) + overhead
      • Target_Coef_A, Target_Coef_B, Target_Coef_C - comes from EPA "Test Car List Data Files"
      • overhead - has to be measured and is the HP equivalent of the vehicle electrical overhead
    • Brake Specific Fuel Consumption - is the fuel consumed per unit of power.
    But any model needs validation or test points to make sure no one has made a mistake.

    I incorporated Wayne's and other open source metrics to validate my 2016 engineering model:
    [​IMG]

    This engineering model is based upon the EPA roll-down coefficients and thanks to the extensive testing, we have the same data for the EPA tested, 2016 Prius family:
    [​IMG]
    The reason for having a model is we can project performance of multiple Prius variants with the changes that the EPA used in their testing. This separates vehicle performance from ordinary, individual variability.

    Now one of the best things about having a model backed up by benchmark data is to diagnose problems with a car. I've done this since 1971 when I bought my first car, a used 1966 VW MicroBus. It is also how I also tell if someone wants to diagnose a MPG problem or just came here to b*tch.

    GOOD LUCK!
    Bob Wilson
     
    #8 bwilson4web, Jul 14, 2016
    Last edited: Jul 14, 2016
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  9. thunderstruck

    thunderstruck Active Member

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    Then press the trip button (the left one) to toggle between A, B, since start, and lifetime summaries.
     
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