1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Difference between actual and reported panel consumption

Discussion in 'Gen 4 Prius Fuel Economy' started by Diego Sausen, Sep 28, 2019.

  1. Diego Sausen

    Diego Sausen Member

    Joined:
    May 29, 2019
    33
    30
    0
    Location:
    Porto Alegre - RS - Brasil
    Vehicle:
    2018 Prius
    Model:
    N/A
    Hello
    This weekend I decided to take a 800 km trip with my Prius.
    I filled the tank until it clicked, then I imagine it was 43 liters and restarted the odometer A.
    After traveling 400 km, I decided to put another 9.5 liters and, when I was almost at home, I put another 4.5 liters.

    Total liters = 57.
    Total distance traveled = 793km
    Estimated autonomy remaining on panel = 114km
    Total km (793 + 114) = 907 km
    Average = (907km / 57l) = 15.91 km / l

    But the panel shows that my average consumption was 5.3 l / 100 km, which corresponds to 18.86 km / l

    Is such a large difference of almost 20% between actual consumption and that calculated by Prius normal?

    Is the remaining range reported (114 km) reported on the dashboard already considering the tank reserve or will there be a few more miles after that?

    Is there an error in my calculation?

    Thanks!
     
    #1 Diego Sausen, Sep 28, 2019
    Last edited: Sep 28, 2019
  2. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,571
    48,862
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    oh dear
     
    Diego Sausen and Raytheeagle like this.
  3. Raytheeagle

    Raytheeagle Senior Member

    Joined:
    Jul 1, 2016
    11,251
    15,474
    0
    Location:
    Bay Area, California
    Vehicle:
    2019 Prius Prime
    Model:
    Prime Plus
    They’re trying to broaden your horizons ;).

    I work with a couple of Brazilians and columbians and when they get going, I just sit back and watch:oops:.

    It gets loud and animated(y).
     
    Diego Sausen likes this.
  4. Diego Sausen

    Diego Sausen Member

    Joined:
    May 29, 2019
    33
    30
    0
    Location:
    Porto Alegre - RS - Brasil
    Vehicle:
    2018 Prius
    Model:
    N/A
    Excuse me.
    I had not passed the text through the Google Translate filter. (n)
    Now I edited for English.(y)
     
    bisco and Raytheeagle like this.
  5. Raytheeagle

    Raytheeagle Senior Member

    Joined:
    Jul 1, 2016
    11,251
    15,474
    0
    Location:
    Bay Area, California
    Vehicle:
    2019 Prius Prime
    Model:
    Prime Plus
    The fuel guess o meter is tough to use as a measure;).

    There are a lot of factors that can explain the variance between the display and reality :cool:.

    I don’t pay it much mind personally as I am getting better mpg than 95% or more of the cars around me:).

    Good luck and keep us posted (y).
     
    bostonbruins8703 likes this.
  6. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    7,041
    7,580
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    Yes, definitely the cause of the error was your "random" fills. The km/l, l/100km or MPG reading is really messed up when it's only partly filled. I've had 4 cars with dashboard displays - and while they're never 100% accurate, they'll all mess up if they're not filled.

    Sometimes a faulty fuel pump/bowser which cuts off early will give a false reading. I can't find where I read it, but the DTE assumes you're filling the tank - not filling causes it to guess!!!

    Oh - and if filled, is quite accurate - relatively.
     
    Raytheeagle likes this.
  7. Mendel Leisk

    Mendel Leisk Senior Member

    Joined:
    Oct 17, 2010
    54,474
    38,106
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Just keep at it, and try to keep your fill up technique consistent. But regardless, after 3 or 4 tanks, an overall calculation will be getting more and more accurate.

    FWIW, my 3rd dash vs calculated is arou 7.5% difference, with what is it now, 160+ full ups.
     
  8. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,571
    48,862
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    how many km on her?

    have you been tracking fuel economy since you got it?

    did you buy it new?

    did you fill to one click each time?

    20% is a big discrepancy, but as stated above, there are variables that must be excluded when measuring.
     
  9. CooCooCaChoo

    CooCooCaChoo Active Member

    Joined:
    Jan 7, 2015
    886
    493
    0
    Location:
    Cow-lee-fornia
    Vehicle:
    2023 Prius
    Model:
    LE
    The tank is now full, and the trip reads as 0 km driven at this point. The 43 L that you pumped shouldn't be taken in consideration because that was pumped to get you to full so you can calculate how many kms were driven per liter of fuel, which is how almost everybody calculates fuel economy. Miles per gallon for us Americans.

    Assuming you filled her up until it clicked:

    400 / 9.5 = 42.1 km / L

    Second fillup on the trip, again assuming you filled her up till it clicked:

    393 / 4.5 = 87.3 km / L

    That's a huge difference between the two fillups.

    But yeah, something wrong with the numbers, and I think it has to do with how you tried to calculate fuel economy. You tried to calculate it for the entire tank and not the distance you travelled, also taking into consideration how much fuel is left as well as DTE.

    How much fuel left and DTE are just guesses by the computer, albeit how much fuel left is a bit more accurate than DTE, which is highly inaccurate and should only be used when you are really pushing the car to empty.

    do not know if you left some numbers out, or remembered the numbers wrong, but I would discard these numbers and wait till another trip to figure your fuel economy.
     
    alanclarkeau and Mendel Leisk like this.
  10. Diego Sausen

    Diego Sausen Member

    Joined:
    May 29, 2019
    33
    30
    0
    Location:
    Porto Alegre - RS - Brasil
    Vehicle:
    2018 Prius
    Model:
    N/A
    Thanks for the answers.
    I am the sole owner of the car (year 2018) and is reaching 8,000 km.

    After filling the tank, I drove about 400 km to my destination (Brazil-Uruguay border) and there I drove another 10 km there.

    The other day, when I was going back to my house, the estimated remaining gas in the tank was 310 km.

    So, as I needed to travel 390 kilometers to get home and didn't know where the gas station would be on the road, I decided to put another 9.5 liters just to travel more safely.

    At that moment, the car began to mark that the range was 430 km.

    The road is very dangerous as it is a simple road with many two way trucks.

    So many times I had to force the engine to overtake - the car had four passengers and its trunk was full of stuff we bought from the freeshops.

    A few hours later, when I was about 50 km from my house, the remaining range was 70 km.

    Then I started to worry if autonomy could plummet and with that I would run out of gas in the car.

    I read here in the forum that letting the car die without gas is a big problem, so I stopped at a post to put another 4.5 liters.

    Today the car marks that I drove 795 km and I have another 111 km of autonomy.

    And he reports that the average consumption on this trip was 5.3 l / 100 km - which corresponds to 18.6 km per liter.

    But the truth is that with 57 liters I will travel 15.90 km per liter.

    Sorry if you got confused, but I found it important to clarify the circumstances
     
  11. Mendel Leisk

    Mendel Leisk Senior Member

    Joined:
    Oct 17, 2010
    54,474
    38,106
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    I like measuring distance traveled using the odometer. When you fill up (always using the same technique), reset a trip meter (your preference, but always the same one) AND note the odometer reading, say on the gas receipt.

    The next time you fill up, again note the odometer reading, and the trip meter (just for a cross-check), on the gas receipt.

    Sure you can use the trip meters. I do, but mainly for feedback, and the cross-check. They are easy to accidently reset, or just lose track of. The odometer is permanent.

    Subract the earlier odometer reading from the last reading: that is your distance travelled. Check that number against the trip meter. Then:

    To determine kilometer per liter:

    Divide kilometers traveled by liters of gas

    Or, to determine liters per 100 kms (the usual in Canada):

    Multiply liters of gas by 100, and divide by kilometers traveled.

    The above is the mechanism of a ratio equation. For example, with 400 kms traveled and 20 liters consumed:

    upload_2019-9-29_10-43-31.png

    Results in fuel consumption of 5 liters per 100 kilometer.
     
    Diego Sausen likes this.
  12. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    107,571
    48,862
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    and do it every time you fill up.

    sounds like you're doing pretty well to me (y)
     
    Mendel Leisk and Diego Sausen like this.
  13. CooCooCaChoo

    CooCooCaChoo Active Member

    Joined:
    Jan 7, 2015
    886
    493
    0
    Location:
    Cow-lee-fornia
    Vehicle:
    2023 Prius
    Model:
    LE
    Yeah, usually almost all of us who keep track of fuel economy will only do FULL fill-ups as that is easier to calculate. What kind of terrain are you driving though? Even roadways? Hills? Mountains? That will have a major effect on fuel economy as well.

    If you find yourself a bit of ways away from home or from a gas station, all you need to do is slow down. The Prius gets phenomenal fuel economy when you are driving slow.
     
    Diego Sausen likes this.
  14. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    7,041
    7,580
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    Refer to my POST #6.

    The "DISTANCE TO EMPTY" (DTE) doesn't work well (or at all) except with completely filling the tank. I understand you might have not wanted to fill - maybe more expensive? - but the DTE can't help with that. Yes, sometimes it'll advance it a bit, but that's only from detecting some change in the Fuel Tank reading, but a wild guess only.

    I can't see how TOYOTA does it - but VOLVO says "This calculation is based off of the average fuel consumption over the past 20 miles of driving along with the amount of fuel left in the tank."

    Consequently, if your driving environment is variable, the DTE will vary considerably - I've seen it go up at times after a long stretch where the fuel economy is very good. A stretch of driving where the engine is working hard will see it plummet after driving like that for a while. Where it was particularly noticeable is over a mountain range - going up, DTE dropped quite a bit - but going down the other side saw it continue to drop - I assume because it's reading distance and assuming I'm still driving hard - but after a few kms downhill, it levelled out, barely moving.
     
    Diego Sausen likes this.
  15. Diego Sausen

    Diego Sausen Member

    Joined:
    May 29, 2019
    33
    30
    0
    Location:
    Porto Alegre - RS - Brasil
    Vehicle:
    2018 Prius
    Model:
    N/A
    Thanks for the answers.

    The way I was doing the calculation was not adequate.

    So this week I will go to the station to fill the tank, and to know effectively how many liters I used (because there is still some gas in the tank).

    This way I will be able to compare effectively with what the panel is marking of consumption.
     
    Raytheeagle, Mendel Leisk and krmcg like this.
  16. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    7,041
    7,580
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    I keep a spreadsheet which calculates l/100km (and UK MPG).
    upload_2019-10-1_7-53-56.png

    If you want the formulas I used - bearing in mind that they're for litres/100km (and UK MPG):
    upload_2019-10-1_7-59-0.png
     
    Diego Sausen likes this.
  17. Diego Sausen

    Diego Sausen Member

    Joined:
    May 29, 2019
    33
    30
    0
    Location:
    Porto Alegre - RS - Brasil
    Vehicle:
    2018 Prius
    Model:
    N/A
    Very good this spreadsheet! Thanks.

    I'll make one for myself too.

    Too bad I didn't get this idea as soon as I bought the car to monitor performance from the first tank.
     
  18. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    7,041
    7,580
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    I've had fuel records since 1975 - spreadsheet since 1993. There are 2 other similar parts of the spreadsheet which document "Capital Purchases & Standing Charges" and "Service and other running charges". They are compiled together at the top.
     
  19. GasperG

    GasperG Senior Member

    Joined:
    May 13, 2011
    1,168
    597
    1
    Location:
    Slovenia
    Vehicle:
    2018 Prius
    Model:
    N/A
    So, all this talk and no info on how much difference between BC and actual consumption. On Gen3 there was 6-8% error, is Gen4 more accurate?
     
  20. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    7,041
    7,580
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    It's about the same - my last 3 cars and ones in my work fleet are similar - and all read slightly optimistically, which probably ties in with the need to keep some fuel in reserve so it isn't EMPTY when it reads EMPTY. The method of calculation takes readings from various sensors - and can't possibly be accurate - but it gives a good guide. I'd far rather it being a few percent out than running out of fuel the moment the needle hit "E".

    As I said above, "VOLVO says "This calculation is based off of the average fuel consumption over the past 20 miles of driving along with the amount of fuel left in the tank."

    SUBARU's manual (Forester) states "...shows only an approximate indication of fuel efficiency ... The driving range on the remaining fuel is only a guide."

    RAV4 says "Use the displayed fuel consumption as a reference only"

    FORD says: upload_2019-10-2_17-16-10.png