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Where does speedo speed come from?

Discussion in 'Model S: Interior & Exterior' started by bxr140, Dec 27, 2014.

  1. bxr140

    bxr140 Member

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    So, when a mommy speedo and a daddy speedo love each other...

    Okay, no...but I'm curious if anyone knows how the speedo readout is derived? Is it similar to most cars where its just based off the output of an rpm sensor? Or is there any GPS (or other magical) correction?

    Context is with respect to non-standard tire sizes, though having clever correction for stock size pertubations like actual manufactured tire size, tread wear, and tire pressure effects seems achievable...no...? On the other hand, stock tire sizes are in the noise between 19 and 21 (and 245/40/20) so perhaps tesla, like other manufacturers, doesn't really care about accommodating non-standard sizes?
     
  2. tom66

    tom66 Member

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    Since there's just a single-reduction gearbox, it's probably just measuring 9.73 (the ratio) x motor shaft RPM and with some multiplier for wheel size. The motor controller needs to know the exact motor position to control the phase sequencing and traction control, so it makes sense to just read this info over the CANbus.
     
  3. Saghost

    Saghost Active Member

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    In addition to knowing the motor RPM, the car knows the speed of each wheel from the wheel speed sensors used by the ABS, TC, and Stability Control.

    If you're not going to use a dedicated sensor or GPS data, I submit that taking wheel speed data makes more sense - especially since by comparing the four data sets you can address wheelspin errors.

    The only advantage of using motor RPM I can see is that the data is likely updated more frequently, and is more precise (but that's a couple of orders of magnitude more detailed than any car displays, and the difference in updates is from tens of times per second to hundreds or thousands.)

    Validation with GPS might make sense, but I'm not aware of any car that does this currently. Being able to "learn" the effective wheel rolling diameter from GPS data on a drive at the touch of a button or periodically would be a nice feature, too, since it changes as tires wear.
    Walter
     
  4. ArtInCT

    ArtInCT Always Learning

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    BXR140:
    I suspect you are concerned that by using a non-standard wheel and tire combo that the speed and odometer reading might be off. Correct me if I am wrong but in both the 19" wheel and tire combo and the 21" wheel and tire combo, the outside radius and diameter of the entire wheel and tire are about the same.
    So if you go to some other wheel, say a 20" you need to match the radius and diameter of that combination to the so called "standard".

    Also, I may be recalling this wrong, but... I remember some screen on the center console that allowed you to choose what size wheels you have mounted...
    again I may be wrong about this....
     
  5. Zextraterrestrial

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    I had smaller diameter rears on for a bit and the speedo was off by ~5 mph at 60.
    so i think it is based off of that somehow/ not gps
     
  6. bxr140

    bxr140 Member

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    Yes, context is non-stock tire sizes, specifically if I go smaller to eek out a little acceleration, or larger to eek out a little more range. Understood on the similarity of stock sizes--they're within a few tenths of a percent (conveniently, so is 245/40/20) which is in the noise considering the other factors that affect rolling dimensions. I was hoping there would be some super secret Konami Code that I didn't know about that could modify the speed signal, even if its just accessible by the service center.

    I come from the world of notoriously awful motorcycle speedometer accuracy--10% or more is practically industry standard, and that gets compounded by the free-for-all modification scene where gearing and tire size changes are all over the map. I almost always install an aftermarket re-calibrator on my bikes--it just modifies the signal between the sensor and the vehicle electronics by some user programmed factor. I wouldn't go so far as to hack up my Tesla to install one...but then again I might...
     

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