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Change tire size to offset speedometer error?

Discussion in 'Gen 3 Prius Main Forum' started by mysticeyes, Jul 27, 2017.

  1. mysticeyes

    mysticeyes Junior Member

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    Hello everyone. I'm the second owner of a 2015 Prius Three in beautiful condition. I've noticed that the speedometer reads about 2 mph higher than speed reported by GPS at ~ 55mph, about a 3% difference. A 205/75-15 tire would offset most of that difference, but tires of that size offer very few choices. Now a 205/60-16 tire is about 2.8% taller, with much better variety in tire designs. I know that somewhere in the basement I have a set of 16x7 wheels, originally off a Subaru WRX with a 5x100 bolt circle and 45mm offset, and that weigh about 11-12 lbs. apiece. I'm thinking about trying a wheel on the Prius to see if it rubs against anything. But I've also read the following in an old thread here:
    "Increasing the rolling circumference in a manual car can improve mpg by reducing engine revs for a given speed. With the Prius transmission this is not the case as engine revs are likely to increase with an increase in torque load that a larger circumference will produce."
    So what do you think? Worth trying?
     
    #1 mysticeyes, Jul 27, 2017
    Last edited: Jul 27, 2017
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    Absolutely.... not.
     
    #2 Mendel Leisk, Jul 27, 2017
  3. WilDavis

    WilDavis Senior Member

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    My advice: Don't worry,:whistle::whistle::whistle::whistle::whistle: be happy! :whistle::whistle::whistle::whistle::whistle:
    …seriously, though you might experiment with the tyre pressures, just remember to set the hinds at 2 PSI less than the fores (…well, that's what is recommended!)
    Here's a link which might be useful:
    Tire Size Calculator - Tire & Wheel Plus Sizing
    Good luck! ;)
     
    #3 WilDavis, Jul 27, 2017
  4. CNYhybrid

    CNYhybrid Member

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    Why are you so sure the GPS is more accurate than you speedometer?

    Posted via the PriusChat mobile app.
     
    #4 CNYhybrid, Jul 27, 2017
  5. mmmodem

    mmmodem Senior Taste Tester

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    Seconded. Positively, absolutely not!
     
    #5 mmmodem, Jul 27, 2017
  6. 4est

    4est Active Member

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    Because IT IS
    GPS is the most accurate speed you can get

    Speedometer is PROGRAMMED to lie
     
    #6 4est, Jul 28, 2017
  7. Mendel Leisk

    Mendel Leisk Senior Member

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    If the speedometer indicates (for example) 50 mph, when in fact you're travelling 48 mph, what are the implications? That you're travelling 2 mph slower than indicated. That's about it.

    Not exactly a big problem, nor insurmountable. You could:

    1. Drive 52 mph, be spot-on the speed limit.
    2. Drive 50 mph.
     
    #7 Mendel Leisk, Jul 28, 2017
  8. xliderider

    xliderider Senior Member

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    OP, OCD much? ;)
     
    #8 xliderider, Jul 28, 2017
    Mendel Leisk likes this.
  9. Prodigyplace

    Prodigyplace Senior Member

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    @bwilson4web did something similar with his Gen 1 Prius, I think. He is our measurement expert and can more intelligently explain his testing.
     
    #9 Prodigyplace, Jul 28, 2017
  10. David Beale

    David Beale Senior Member

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    Well, it's the -odometer- that is important accuracy wise. I suspect you will find it is almost right on. As posted above, speedometers are often set high intentionally. To keep you from getting -as many- tickets I suppose. Larger tires can certainly offset the speedometer error, but they can actually be illegal in -some- jurisdictions.
     
    #10 David Beale, Jul 28, 2017
  11. Mendel Leisk

    Mendel Leisk Senior Member

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    I'm not checking my facts, but I believe the purpose was to try to slow people down, reduce injury severity, and fatalities. You never want to have a speedo that reads low, deadly accurate is an impossibility, so reading a bit high was mandated.
     
    #11 Mendel Leisk, Jul 28, 2017
  12. MelonPrius

    MelonPrius Senior Member

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    Larger tires would fix the speedometer issue. But I believe it would then make the odometer less accurate. The actual mpg (both hand calculated and with the computer) would be ~3% lower because the car would travel further than the odometer would indicate.
     
    #12 MelonPrius, Jul 28, 2017
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