Why can’t GPS be used to calibrate the speedometer?

[Goobers]

So, there was something I read (or saw?) a while ago, don’t remember if it was an article, forum post or even YT video, but the basic idea is this:

The manufacturers, not just Tesla, purposely report a higher speed on the speedometer than you are actually going so that even if your speed varies a little high, it won’t be high enough for a speed enforcement officer to tag you as speeding. Most places allow up to 5 mph over the speed limit before tagging you, but there are a few places that don’t allow any.

I’m inclined to believe it on just the basis that people (including myself) vary in speed under and over what they are trying to stay at. And even cruise control sometimes varies too, for whatever reason.

This isn’t just the manufacturers “looking out for you,” as it helps absolves them of the issues where drivers claim “well, my speedometer said I wasn’t going that fast,” trying to push the fault onto the manufacturers.

 

[FastEddieB]

It’s simpler than that, I think.

If a speedometer is a tad optimistic, it has you going into turns a bit more slowly than you think you’re going. Not much harm in that.

But if a speedometer is pessimistic, the inverse is true - you’re going faster than you think you are, and harm can come of that.

So I recall manufacturers are given a bit of latitude for speedometers reading a little fast, but none for reading slow.

 

[DanDi58]

DOT regulation 383.83 states that a speedometer must be accurate to within plus or minus 5 mph at 50 mph. So the manufacturers have nothing to do with it.

 

[hgmichna]

Tesla cars calibrate their speedometer from GPS.

How do I know? There is no other way to do it reliably. There is also no simpler way. And we know that the Tesla speedometer is always accurate to 1 mph or even to 1 km/h, by comparing it to a GPS device.

 

[Goobers]

There is no other way to do it reliably.

You mean except through the process of using how fast the motor spins, multiplied by the drive/final ratio and then the tire diameter?

Of course there’s the occasional times where GPS might lose the signal and get errors because of processor delays…

Your “proof” is based on an assumption.

 

[QtownTaxiGreg]

Pretty sure it's disproved by observation.