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Inaccurate MPG?

Discussion in 'Prius c Fuel Economy' started by Gasoholic, Mar 1, 2017.

  1. Gasoholic

    Gasoholic Junior Member

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    I have read where many state that the MPG reported by the Prius C is inaccurate by 4-8% showing a higher mpg than actual, if calculated by actual fuel burned, checked when refueling.

    So, I did a run of 366 miles, combined in town and highway, with about 320 of this being highway at 60-70 mph. The Trip 1 used for that test, showed 55.1 mpg when I filled up. I put in 6.4 gals of gas, letting it automatically shut off, as I had at the last fill up, no topping off. This computes to 57 mpg, which is higher than stated by the vehicle display by 3.5%, not lower by my calculation.

    Of course there is a possibility that the auto-shutoff did not put precisely the same amount of fuel in, so that the calculation show this difference. But it would seem to me that the accuracy of the vehicle computer is pretty close on the 2016 Prius C3, from my test.

    So, have you all been seeing errors that are not consistent with this?
    Gasoholic
     
  2. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    Always have.
    Always will.

    My (work car) 2010 always favors the house by 5-10 percent.
    I don't fret about it much since I always hand calculate at the pump and determine my real world MPG tank by tank, rather than trip by trip.

    I'm not sure if the G4s have this same bias, but Priuses are somewhat infamous for having 'fake efficiency' displays.

    AS ALWAYS.......
    YMMV
     
  3. Mendel Leisk

    Mendel Leisk Senior Member

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    With liters per 100 kilometer system, lower numbers are better. As @ETC(SS) says, always in favour of the house:

    upload_2017-3-1_10-34-39.png
     
  4. Gasoholic

    Gasoholic Junior Member

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    Maybe it is the newer 2016 Prius C that is just reporting with more accuracy? Anyone out there have a 2016 or 2017 model with information?
    Gasoholic
     
  5. bisco

    bisco cookie crumbler

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    you need more data. it doesn't fill up the same each time. sometimes i get more, sometimes less, sometimes right on.
     
  6. Sean Nelson

    Sean Nelson Active Member

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    I've always seen the car report around 5-7% less fuel usage than it's actually consumed as measured by litres pumped. But that's based on many data points - the individual points vary because you can't rely on the gas pump's auto-shutoff to always fill the tank to exactly the same level. I've seen numbers ranging from as far as 2% pessimistic (car reports more than actual fuel consumption) to 11% optimistic (car reports less than actual fuel consumption). But those outliers are unusual - almost all my fill-ups are in the 4 to 8% optimistic range.

    Of course it's entirely possible that the algorithm has been changed in the 2016 Prius C, so it will be interesting to see what your experience will be like be over several fill-ups.
     
  7. Sam Spade

    Sam Spade Senior Member

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    This.
    Mine has always calculated lower than the display no matter how I do it. So does my Fusion.

    I think it is probably more accurate if you fill to the second click, using a reduced flow rate after the first auto-stop.
     
  8. Gasoholic

    Gasoholic Junior Member

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    I will keep an eye on it and check the fuel pump each time. Hard to be sure you get the same amount of fuel of course, just about impossible. But if you do it the same way each time, one can hope to get close.
    As I get more fill ups, I will post what I find.
    It is possible that the later models have a revised system, but maybe I just need more data points.
    Gasoholic
     
  9. Mendel Leisk

    Mendel Leisk Senior Member

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    The more fillups you do, the less pump vagaries come into play. Even by two or three fillups, the accuracy is getting really good: if your pump shuts off a bit early one time, it balances out on the next. Our third gen: a tediously regular 7.5% bias. The Toyota BS factor...
     
  10. marlinsmobile

    marlinsmobile Active Member

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    Like everyone else, my 2013's displayed MPG runs about 6% higher than calculated at the pump. I have never experienced a tank where the display was lower than the calculated MPG (my data is here).

    It will be interesting to see what happens as you gather more data.
     
  11. fuzzy1

    fuzzy1 Senior Member

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    Please update your report when you have about 10,000 miles of data on this.
     
  12. wjtracy

    wjtracy Senior Member

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    Yes I have the data for every fill on our Prius. Before that I was out of the habit for about 20 years.
    Of course Gen2 has the infamous Guess Tank.

    I actually don't even record the Prius number on the MFD.
     
  13. Mendel Leisk

    Mendel Leisk Senior Member

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    I do, but just because I don't want to "break the chain" in my spreadsheet.
     
  14. MoarMPGplz

    MoarMPGplz Junior Member

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    FWIW, I used to drive a 2012 Toyota Prius-c Two, which I had almost 50K miles on it and 111 fillups. The MPG, hand-calculated, was almost always 7-8% higher than what was being reported by the car's MFD. I now have a 2016 Toyota Prius-c Two, and have over 10K miles on it now...24 fillups to date. I now get the opposite results where the MPG, when hand-calculated, is about 4% lower than what is being reported by the car's MFD. I always tried to fill up about a gallon past the first click-off point to get more accurate tanks, though sometimes I 'top off' a bit less if I saw the gas reaching the top of the filler tube. I just rely on the hand-calculated results anyway, so not too worried about the discrepancy...I got 56 MPG in my 2012 Prius-c and currently get 54 MPG in my 2016.
     
  15. Sam Spade

    Sam Spade Senior Member

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    Not unless you go to the same pump at the same station all the time.
    Fuel splashes around as it goes in at a fast rate......and the auto-shutoff will trip prematurely sometimes.
    Going to the second "click" is better but a long term average is best; like at least 5 fillups.
     
  16. Gasoholic

    Gasoholic Junior Member

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    Good thought, I will try letting it shut off twice. I knew that it was possible that the gal level was not exactly the same, though I figured it was close. But even if it held a little more gas, the mileage would still have been close, as the mileage was 2 mpg higher than the Trip 1 results. But as I get more information on this, it should become clear whether the mileage is off and by how much,
    Thanks,
    Gasoholic
     
  17. Mendel Leisk

    Mendel Leisk Senior Member

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    As long as you use the same fill technique each time, your calculations will be fairly accurate, and again: any vagaries in a single tank calculation will balance out in the next tank. Or two fillups, at the most. Pumping gas up to the neck is a fools game, can mess up the gas fume control system (expensive), and just ticks off the next guy in line.

    My 2 cents: fill till it shuts off, done.
     
  18. Sam Spade

    Sam Spade Senior Member

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    Nobody in this thread suggested that you do anything like that.

    If the nozzle is still fully inserted, going to the second "click" will NOT cause an overfill condition.
     
  19. Sean Nelson

    Sean Nelson Active Member

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    Actually, MoarMPGplz claimed to do exactly that, so I think the caution about messing up the evaporative emission control system is well warranted.
     
  20. Sam Spade

    Sam Spade Senior Member

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    OOPS. Sorry I failed to read to the end of his post.
    You are right.....HE is dead wrong......this practice is not only dangerous because of potential spills but it can result in an expensive repair bill......AND it accomplishes exactly NOTHING useful.
     
    Mendel Leisk likes this.