Speedometer shows 2 MPH faster than actual speed

Discussion in 'Gen 3 Prius Technical Discussion' started by windmilljumper, Sep 29, 2009.

  1. radioprius1

    radioprius1 Climate Conspirisist

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    15 inch rims. My speedo is 2mph under what police radar setups show.
     
  2. cmalberto

    cmalberto New Member

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    I wonder if this is also the reason why the puter MPG is higher than actual calculated. I suspect so. I haven't checked via a GPS yet.

    --Mickey
     
  3. ken1784

    ken1784 SuperMID designer

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    The MPG number is based on two parameters, trip distance and fuel usage.
    The "trip distance" is fairly accurate.
    It looks the "fuel usage" is intentionally biased 5%.
    The speedometer display is also intentionally biased to meet the regulation.

    Ken@Japan
     
  4. windmilljumper

    windmilljumper New Member

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    Nice to hear that. I have 17" tires and am 2 mhp off constantly. I have 3300 miles on mine. Looks like some adjustments should be possible
     
  5. Jon S

    Jon S Member

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    you guys know when the tire tread wears down, the speedometer reading is more accurate, right? When the tread is almost gone, the speedometer will be about spot on.... as the circumference changes, the distance traveled is reduced by 4% (or roughly 2 MPH at 50MPH) as it wears out.
     
  6. ken1784

    ken1784 SuperMID designer

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    No. It's opposite.
    When the tire tread wears down, the distance traveled is increased, then the speedometer offset is also increased.

    Ken@Japan
     
  7. Jon S

    Jon S Member

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    Oops... I stand corrected... I got it backwards.... stupid math....
     
  8. jdcollins5

    jdcollins5 Senior Member

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    This thread has been around for some time but I just learned how to get in to the Service Menu on my Gen III. One of the items on this service menu is GPS and Sensors page. One of these sensors is the speed sensor.

    While driving to work this morning with the CC set at 60 mph, the Service Menu page displayed the reading of the speed sensor to be 57 mph. So my car is actually biased by 3 mph.
     
  9. hyo silver

    hyo silver Awaaaaay

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    After installing new tires with a slightly higher rpm than stock (863 vs 855), my wife can assure you the speedometer is calibrated perfectly, right up to 136 kmh. ;)
     
  10. saechaka

    saechaka Member

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    Can you state what tires and size you are using? This 2mph off reading has always bothered me. I've tracked the actual distance traveled and that was off too. Came out to be about 1 mile off for every 50 miles. Which means the mpg reading would be off too for many people. Thanks
     
  11. hyo silver

    hyo silver Awaaaaay

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    I replaced the stock Goodyear Integrity (185/65/15) with Goodyear TripleTreds (195/60/15). I would highly recommend a switch for improved traction, handling, and safety, but not just to fix the built-in speedometer error.
     
  12. PriusDreamer

    PriusDreamer Member

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    I have a package V , no options, 17" tires and I am also 2 mph off at 60. Two GPS's show me traveling 58 MPH. Not a huge deal, but something to be aware of.
     
  13. ken1784

    ken1784 SuperMID designer

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    I believe the 195/60R15 (863rev/mile) tires turn more than 185/65R15 (855rev/mile) at the same speed, therefore the speedometer reading with 195/60R15 is higher than with 185/65R15.

    We need larger diameter (low rev/mile) tires for less speedometer error.

    Ken@Japan
     
  14. saechaka

    saechaka Member

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    True. I'm using the Goodyear Comfort Tread 195/60R15 and it's still 2mph off. I'm closing to needing my second set of new tires, 175k currently, and will probably go with the Goodyear Fuel Max. I'm just not sure what size.