Where does speedo speed come from?

[bxr140]

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?

 

[tom66]

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.

 

[Saghost]

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.

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

 

[ArtInCT]

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....

 

[Zextraterrestrial]

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

 

[bxr140]

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.

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...

 

[RenMan68]

Interesting update here... since I got new tires I've been tracking the tread wear on each tire individually with a tire tread depth gauge. I've had this new set for 14,400 miles now and I'm getting a warning from my model 3 saying that the rear tire tread depth is low. The average rear tread depth is 0.218" (the legal minimum in the USA is 0.063") so it's not accurate that I need new rear tires. The front ones have 0.312". Apparently using regenerative braking has a tremendous effect on tire wear - about a 4x difference! Anyway, I'm wondering if the error message is somehow a result of the car knowing that the rear tires are consistently spinning faster than the front ones (since it monitors all 4 wheel speeds), since they're smaller now, and that's generating the error/warning. Any thoughts?

 

[DOCAL]

Service forgot to re-attach my GPS antenna when I got the MCU upgrade, so my car was showing stationary in Tesla HQ for a week, speedo worked just fine during that time. That doesn't mean that GPS couldn't also be used as a backup, or for calibration or adjustment, however there must be a sensor on the drive train somewhere.

 

[RenMan68]

Service forgot to re-attach my GPS antenna when I got the MCU upgrade, so my car was showing stationary in Tesla HQ for a week, speedo worked just fine during that time. That doesn't mean that GPS couldn't also be used as a backup, or for calibration or adjustment, however there must be a sensor on the drive train somewhere.

It seems that ABS needs to have speed sensors on each wheel so that it can tell if one of them has "locked up" by having zero speed when the other three are still moving. The speed display is probably an average of the four wheel speed sensors. GPS would be very hard to use for accurate car speed since the only way it could do it is to calculate the speed by dividing the distance traveled over some time interval. This leads to considerable errors when the car is turning. The location accuracy of GPS is roughly fifteen feet or so; imagine making a sharp turn at a slow speed with an error on the scale of the car itself. The errors would be enormous. GPS signals also deteriorate with obstructions, so driving through a canyon (where the orbital view is limited) and trees curving over the roadway - the car would have really terrible GPS accuracy and might lose it all together. Using GPS for speed is really only useful / necessary for vehicles where speed through the medium and speed over ground are different, like boats and airplanes where the current (water or air) is considerable and needs to be removed from a calculation that determines an arrival time. (it can also course correct to remove a cross-wind or cross-current).