Speedometer off???

Discussion in 'Prime Technical Discussion' started by blanka98, Oct 29, 2018.

  1. blanka98

    blanka98 Junior Member

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    Has anyone notice their speedometer off? I use Waze quite often and my speed is off by 3mph. I have a 2018 Prime advanced. TIA


    iPad ?
     
  2. Tideland Prius

    Tideland Prius Moderator of the North
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    If it's over, it's fine.

    Most cars (practically >90%) are fitted with speedometers that read slightly over with the degree of error depending on the type of vehicle (sports cars vs. regular sedans).

    Mostly it's a liability issue. No manufacturer wants to be responsible for the speeding ticket because the speedo reads lower than actual, so they play it safe and make it read higher. There is one other reason I can think of but it's not that relevant to Prius.
     
    #2 Tideland Prius, Oct 30, 2018
    Last edited: Oct 30, 2018
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  3. davecook89t

    davecook89t Active Member

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    If it reads lower than actual, wouldn't that result in more speeding tickets?
     
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  4. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    I think @Tideland Prius probably meant to say speedos tend to read high, just a typo.

    Good catch btw, it all sounded good to me lol.
     
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  5. Tideland Prius

    Tideland Prius Moderator of the North
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    D'oh! Sorry, yes. It reads higher than actual. I'll edit the post.
     
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  6. blanka98

    blanka98 Junior Member

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    I am just concerned more about not getting the true mpg specially if you run EV. It is rated to run 25 miles on full charge but with the discrepancy you can be getting roughly 23 miles


    iPad ?
     
  7. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    The speedometer reads high, to discourage speeding. The odometer, on the other hand, should be reasonably accurate. The car is purposely exaggerating one, but not the other.

    Another thing the Prius tends to exaggerate is the mpg, in my experience by around 7%, on the optimistic side. It is at least relatively consistant in the exaggeration, so you can keep an eye on displayed mpg, just keeping mind that actual mpg will be lower. You can track the actual mpg, using the odometer readings (miles travelled) and gallons of gas added at fill ups.

    Do mpg calculation for a few tanks, compare to what the car's telling you, and you'll know how much it exaggerates.
     
  8. Usle

    Usle Member

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    What size tires, I find with my 195/65-15 tires my speedometer is correct, according to my Garmin.
     
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  9. Salamander_King

    Salamander_King Senior Member

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    Don't know at higher speed, but on my PRIME at ~30mph, speed displayed is identical to the roadside radar that displays "Your Speed is". Although, it is possible those roadside radar also reads higher than actual to discourage speeding.
     
  10. CraigCSJ

    CraigCSJ Active Member

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    Are you sure the Prius exaggerates mpg? I believe the calculated mileage may be lower than the actual mpg due to errors in the pump gallon gauge. Once I took an exact gallon jar and it was not quite filled when the pump said one gallon. While there is usually a difference between calculated and Prius indicated, either or both could be wrong. For me, I think an average of the two is the most accurate.
     
  11. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    If the stations pumps were consistantly off, by 7%, there'd be hell to pay, lol.

    With my 3rd gen Prius at least, the error jumps around but on average is a very consistant 7%.

    Wayne Gerdes, at CleanMPG, had a pre-production 4th gen, and it's displayed mpg more-or-less matched calculated mpg. I believe the production vehicles though, continue to display "rosy" mpg.

    I've logged all the miles travelled, and all the gas added, virtually since our car rolled off the dealership lot, currently152 logged fill ups.

    upload_2018-11-7_17-7-49.png
     
  12. CraigCSJ

    CraigCSJ Active Member

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    My Prius Prime has traveled 17706 miles since new, and its gauge says its mpg over that distance is 150.1 mpg. I have filled the tank 18 times and according to the gas pump gauge put 123.086 gallons of gas into the car. This calculates to 143.9 mpg. I believe my actual mpg is likely around 147 mpg.
     
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  13. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    Your calcualted vs displayed is around 4.3% difference. I would think that is the error. Not sure why you would want to split the difference.

    Here's a couple of years of my 3rd gen, fill-by-fill:

    upload_2018-11-8_7-51-42.png

    I really think this can laid on Toyota's doorstep. They are doing this to make owners feel better about their mpg, not that they need to, they're pretty much the industry leaders.

    When I first read your mpg I thought it was a typo lol, then realized: a Prime owner. Anyway, all the best.
     
  14. CraigCSJ

    CraigCSJ Active Member

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    I split the difference because I believe that both measurements are likely to be wrong and the combination of two data points is closer to the truth than picking one of the data points.
     
  15. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    We'll agree to disagree. Again: if gas stations were routinely short changing consumers by 4~5%, there'd be hell to pay.

    The smoking gun in the room: Toyota gets rave reviews and owner testimonies that are rosier than reality.

    FWIW, our previous Honda Civic Hybrid displayed mpg was always spot-on or slightly under-reporting (saying mpg was slightly worse than calculated). It can be done. That's about the only nice thing I can say about that car though.
     
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