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Speedometer: Inaccurate?

Discussion in 'Model S: Driving Dynamics' started by brianstorms, Oct 29, 2013.

  1. brianstorms

    brianstorms Member

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    #1 brianstorms, Oct 29, 2013
    Last edited: Apr 11, 2014
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  2. ElSupreme

    ElSupreme Model S 03182

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    Those side of the road speed signs are rarely calibrated. And most of the time very poorly setup. They are not reliable measures of real speed.

    The best way to check is with a GPS device, or use a GPS app on your phone.

    I have only seen 1mph different using this method. And normally when gaining/losing elevation, which would account for the difference. I think the Model S (at least mine) has a pretty dead on speedo. Although I haven't done any rigorous testing this has been my casual observations.
     
  3. Raffy.Roma

    Raffy.Roma Active Member

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    @brianstorms

    It's normal for speedometers of any kind (also digital speedometers) to have a tolerance of some percentage. In your case the tolerance of the speedometer looks to be 1%. It's good. it's not a matter of calibration IMO, but of accuracy of the measuring device (speedometer) which is in the normal range of all speedometers (tolerance of 1%).
     
  4. Denarius

    Denarius Active Member

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    All of my other cars have been off by about 4-5mph at 75. My Model S is off by 1mph. Check it with GPS. Radar has to be calibrated for the atmospheric pressure, only laser is accurate all the time.
     
  5. Jeeps17

    Jeeps17 Cath Jockey in a P85

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    FWIW, when I drive past similar fixed radars the reading on my speedometer is always identical.
     
  6. Doug_G

    Doug_G Lead Moderator

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    This. They put one of these gadgets up in my neighbourhood periodically, and it always matches within 1 kph.

    I did a charity drag race event this summer where they used a radar gun to measure top speed. They measured 203 kph. My speedometer reported 203 kph (I have it on video!).

    The Model S speedometer is by far the most accurate I have ever seen.

    Could easily be poor calibration of the roadside sign gizmo. Also bear in mind that if the radar is well off to one side, it will get a cosine error and report a speed lower than actual.
     
  7. AmpedRealtor

    AmpedRealtor Active Member

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    My speed is usually within 1 MPH of those signs the couple of times I looked, but they are not meant to be totally accurate.
     
  8. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    On mine the speedometer shows 1 mph higher than the curbside radar. I've been through a number of different ones in the various cities around here, and they all read the same.
     
  9. rolosrevenge

    rolosrevenge Dr. EVS

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    Well according to Broder when the speedometer says 54 you're really doing more like 60 :rolleyes:
     
  10. brianman

    brianman Burrito Founder

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    Zing!
     
  11. Red

    Red Member

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    Actually, GPS and actual radar will always show about 7% lower speed than your speedo on any certified production car becuase your tacho is supposed to lie to you by that much, to keep you on the safe side of the law. Add to that the tolerance of speed traps and you can actually speed over the limit by some 10-15% safely. Not suggesting it, just saying.

    This is normal. Your tacho is OK when it shows those 7% over actual speed. In any car.
     
  12. Doug_G

    Doug_G Lead Moderator

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    Except Model S. The speedometer is accurate.

    The Roadster is a different story. The main speedometer is very inaccurate, and actually not consistent, but the apparently GPS-calibrated speedometer on the touchscreen is always spot on.
     
  13. ZBB

    ZBB Emperor

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    I don't have a direct link to the DOT regulation, but the regulation allows speedometers to be inaccurate on the high side, but cannot report you going a slower than actual speed. i.e. if you are actually going 50, it can display 50, 51, 52, etc. But it cannot display 49. Most manufacturers build in a % error to make sure they are not violating the regulation. In past cars, I've seen 1-4 mph over-reported when going 60 (confirmed by GPS that was stated to be accurate to +/- 0.1 mph). The Tesla appears to be 1 over (although I no longer have that GPS to confirm...).
     
  14. N4HHE

    N4HHE Member

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    The DOT spec is +- 5 MPH at 50 MPH. Pretty darn sloppy.

    The EU spec is -0% +10%. In Europe a speedometer may not indicate slower than the vehicle is actually moving.

    - - - Updated - - -

    Basic trigonometry. Your speed on a vector headed to one side of the radar sign will always be less from the perspective of the radar sign than had you been headed straight for it.

    If I was designing a radar sign I would make provisions to enter the angle and distance for calibration. But far too often those erecting such signs don't understand such things and wouldn't make the calibration even if it was available.
     
  15. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    I'm pretty sure the DOT regulations date from bias-ply tire days. Bias-ply tires can easily vary their RPMs by 30% just from inflation differences. A ten MPH range is actually quite tight given that. Of course it would have been better to update the regulations but I doubt there have been many complaints.
     
  16. mknox

    mknox Well-Known Member

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    Yep. There's one in a school zone near my house and it is always spot on with my speedometer.
     
  17. ken830

    ken830 Model S (Res#P12,447)

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    Car and Driver did an article on speedometer accuracy more than a decade ago...

    http://www.caranddriver.com/features/speedometer-scandal

    Basically... many cars have speedometer reads faster than reality. In general, European cars are the most exaggerated followed by Japanese Cars. The most accurate are American cars.

     
  18. neroden

    neroden Happy Model S Owner

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    It's your local radar signs which are inaccurate. We have similar radar signs in Ithaca and they report exactly the same numbers as my speedometer.
     
  19. ZBB

    ZBB Emperor

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    thanks for the correction... It must have been the EU reg I was thinking of...
     
  20. Musterion

    Musterion 18h 03m 37s −24° 23′ 12″

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    Does the speedo readout from the streaming data in the phone app when tracking the car use GPS or the car speedo as its source? If GPS you could start there to compare.
     

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