How accurate is the dashboard MPG?

Discussion in 'Gen 4 Prius Fuel Economy' started by TKW_NOLA, Sep 16, 2021.

  1. TKW_NOLA

    TKW_NOLA New Member

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    Newish 2021 XLE now at over 4000 miles on ODO, and total MPG shows at 61 mpg on the instrument panel. How accurate is that likely to be? Is there a correction/fudge factor? Lots of highway miles including a fair amount of 70+ mph. Seems too high, right?
     
  2. davecook89t

    davecook89t Senior Member

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    With our 2 Prii (Gen 2 and Gen 4), the MPG recorded by the car is about 5% optimistic, compared to what I see when I plug our fuel consumption and miles into Fuelly.com. Fuelly shows the MPG for the average 2021 Prius Liftback is 52.3 MPG, 17% below what your car is showing. Possibly your car was driven mostly in slow rush hour traffic on the highway, which would account for the higher than expected MPG on the dash. Both our cars do best MPG-wise when traffic is moving slowly but steadily on the highway, say 30 to 40 MPH. I believe others have found the same.
     
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  3. alanclarkeau

    alanclarkeau Senior Member

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    There's been lots of threads on this - they're not particularly accurate.

    A number of reasons - the first is the physical limitation of doing averages of averages by the computer, plus the inaccuracy of the sensors which read it. Even distance is impossible to get correct - there is no mechanical/electronic speedo which is 100% accurate - depending on tyre wear and pressures, and diameter which changes slightly as speed changes.

    But - I suspect, most manufacturers put it there as a token - not as a mathematical/scientific tool, but for owners to have a fun glance at from time to time. Many people I know say "I didn't even know it was there" - till pointed out.

    They are far more accurate than the first I say about 20 years ago.

    I have records of fuel use in cars since 1974 - using the calculator method with fuel price, total price and ODO reading. BUT - that also uses ODO reading - which, unless corrected, won't be accurate either.

    Personally - with my last 4 cars, I worked out by using that method roughly how accurate it was with the first few fills - mine was 7% (optimistic) with this car, my last 2 cars were about 6 or 7% optimistic.

    Close enough - to not care.

    EDIT - check forums for all brands - BMW, VW, MERC, JEEP etc etc - they all say they're optimistic.
     
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  4. sam spade 2

    sam spade 2 Senior Member

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    Don't guess.
    And don't blindly accept answers from places like this.

    Do the math yourself.
    Miles driven divided by gas used........over several fillups.

    Most owners find the display to be a bit high; up to as much as 10%.
     
  5. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Calculate a few tanks, post the results?

    My third gen is optimistic by around 7.5%.
     
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  6. FuelMiser

    FuelMiser Senior Member

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    What does knowing "total MPG" do for you, whether it's accurate or not? On the other hand, I find the diary page useful for tracking MPG trends, whether or not the individual #s are accurate, at least you can quickly spot a trend if something is going wrong with the efficiency of the vehicle.
     

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  7. JosephG

    JosephG Active Member

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    I believe these gauges calculate your fuel economy based on the fuel injecting. They do tend to be 5-10% optimistic, but OEMs have countered that if you were burning pure gasoline (with no ethanol), that would make them almost perfectly accurate.

    Since the energy of gas per unit volume varies based on the fuel blends and temperature, the onboard computer is probably a better measure of how efficiently you're driving than the actual MPG. If you're doing it for accounting expenses you obviously can't count on it.
     
  8. Mendel Leisk

    Mendel Leisk Sand Pounder

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    I've never heard Toyota, or any of the others, comment on the accuracy of their mpg displays. I doubt running pure gas would effect the accuracy either, rather it would continue to display (relentlessly) optimistic. The motive is pretty obvious.
     
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  9. alanclarkeau

    alanclarkeau Senior Member

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    Here in Australia, I almost always use pure petrol rather than E10. And it's still optimistic as my previous post said.

    TOYOTA's App here gives a discount to owners for UNLEADED, but not E10 - making it cheaper.
     
  10. JosephG

    JosephG Active Member

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    This statement was from GM, but all MPG meters use a similar system. It was discovered by some automotive magazine that Ford MPG meters were as much as 20% off and GM was explaining it's calculated based on injection timing and manifold pressure, so the in-car MPG monitor is estimating energy consumption rather than counting gallons of gasoline. Obviously Ford had made a mistake in their algorithm for calculating MPG somewhere.

    Incidentally, my in-car number is within 4% of my Fully number. Occasionally I get a tank that's more like 7-10% off, but it balances out at the next tank. I wonder if it's because my driving cycle is very high efficiency to begin with that it is closer, the difference seems to be higher in the winter.
     
    #10 JosephG, Sep 22, 2021
    Last edited: Sep 22, 2021
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  11. Mendel Leisk

    Mendel Leisk Sand Pounder

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    You’re looking everywhere but the obvious: Toyota likes to exaggerate.
     
  12. alanclarkeau

    alanclarkeau Senior Member

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    Change tyres - and you could find it different again. There are too many factors involved - it'll never be 100%, it's just a toy indication
     
  13. alanclarkeau

    alanclarkeau Senior Member

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    TOYOTA - nope, every car manufacturer - check the owner's blogs, they all say the same. There are too many factors involved - it'll never be 100%, it's pretty close, taking into account the variabilities. Tyre temperature, the pressures the owner bothered to (or not) check, fuel temperature - it's just a fun indication to keep drivers occupied.
     
  14. Mendel Leisk

    Mendel Leisk Sand Pounder

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    That toy manages to be consistently biased towards optimistic, anywhere between 5 and 10 %, and in my spreadsheet settled down and never budged from 7.5%.
     
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  15. alanclarkeau

    alanclarkeau Senior Member

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    & your speedo remained optimistic too. Part of the equation. Not sure about the ODOMETER - hard to tell, but one test seemed almost 100% accurate. When I changed tyres recently, the speedo changed - even more inaccurate.
     
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  16. fuzzy1

    fuzzy1 Senior Member

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    Speedometers have an intentional built-in bias that is not included in the odometer. The speedometer bias is to prevent the 'speedometer error' excuse for speeding. And it seems to vary by world market region.

    But a similar bias cannot be placed on the odometer in the U.S. without inviting product liability lawyers to file class action lawsuits for warranty fraud. I have been part of the covered class in two such broad suits, despite my cars not displaying the alleged condition. The lawyers collects million$ in fees. The lead plaintiffs get a few thousand dollars each. I was in the bottom rung, getting a 2% mileage warranty extension, which I was very very long past.
     
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  17. Nico3d3

    Nico3d3 New Member

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    On my 2017 base gen4 Prius, the dashboard overestimate fuel economy by around 3 mpg. How I know? I started with a full tank and when I refueled, I noted mileage since last fuel up and how much fuel it took to fill it up
     
  18. alanclarkeau

    alanclarkeau Senior Member

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    You actually need to do a few fills, recording mileage, amount of fuel. And some maths - not sure about MPG - haven't used that for ½ a century - it's odd, as it works backwards. Monitoring it continually will give a much better idea than just one tank - like, full isn't always full, depends on when the bowser clicks off, the angle, temperature of fuel etc.

    I've records of (almost) every fill since 1974.
     
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  19. PaulDM

    PaulDM Active Member

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    Now that’s impressive
     
  20. Kramah313

    Kramah313 Active Member

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    What’s interesting to me is that when the gen 4 was being tested there were posts here saying that the computer was actually underestimating the MPG. In the production release it goes back to overestimating as always. To me, that points to the intentionally overestimating hypothesis.

    If you think about it, a lot of people don’t do actual calculations. So having the computer be under the actual is going to make the car looks worse to the casual driver that just looks at that display and feels better or worse about their purchase of a fuel efficient vehicle. My guess is that it’s set intentionally a bit high so that there’s no chance of it being low, kind of like the speedometer is. But that’s just a guess.
     
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