# Speedometer accuracy - Model 3



## android04 (Sep 20, 2017)

I hope the new map data has updated speed limit info. Most highway speeds in Nebraska increased by 5 MPH about a year ago and the Tesla data still has the old speeds. So it sucks using Autopilot on restricted roads because it only allows 5 MPH above what it thinks is the limit. Resulting in only being able to go about 3 MPH below the actual limit since the Model 3 reads about 3 MPH higher than reality.


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## MelindaV (Apr 2, 2016)

android04 said:


> since the Model 3 reads about 3 MPH higher than reality.


did you change the tire size? you are the only one I've heard say the speedometer isn't accurate.


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## Dr. J (Sep 1, 2017)

android04 said:


> I hope the new map data has updated speed limit info. Most highway speeds in Nebraska increased by 5 MPH about a year ago and the Tesla data still has the old speeds. So it sucks using Autopilot on restricted roads because it only allows 5 MPH above what it thinks is the limit. Resulting in only being able to go about 3 MPH below the actual limit since the Model 3 reads about 3 MPH higher than reality.


Crowd-sourced internet misinformation seems to show the Model 3 speedo reads ~2% high, which would be about 1.5 MPH at highway speed.


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## GDN (Oct 30, 2017)

So how do we know that the test subject you compared the car too is accurate? I once had a "speedometer" app on my iPhone I guess I can fire it up. I rarely use Google Maps on my phone, but does it by chance show your current speed if you are using it for navigation. Could compare to that.


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## android04 (Sep 20, 2017)

MelindaV said:


> did you change the tire size? you are the only one I've heard say the speedometer isn't accurate.


I have the original 18" tires. I based that on readings from my Blackvue dashcam that has GPS speed displayed in the video, smartphone GPS speed apps, and from what I've seen on those radar speed limit signs. They all show 2-3 MPH lower than what my Model 3 reads when I'm on cruise control.


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## shareef777 (Mar 10, 2019)

android04 said:


> I have the original 18" tires. I based that on readings from my Blackvue dashcam that has GPS speed displayed in the video, smartphone GPS speed apps, and from what I've seen on those radar speed limit signs. They all show 2-3 MPH lower than what my Model 3 reads when I'm on cruise control.


That's standard across the auto industry. Most vehicles still have analog gauges so determining exact speed (within a mph) isn't really that necessary.


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## MelindaV (Apr 2, 2016)

shareef777 said:


> That's standard across the auto industry. Most vehicles still have analog gauges so determining exact speed (within a mph) isn't really that necessary.


here's the language from HTSB


NTSB said:


> Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).


so the regulators are good with a +/- 5MPH at 50MPH speeds.


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## Dr. J (Sep 1, 2017)

MelindaV said:


> here's the language from HTSB
> 
> so the regulators are good with a +/- 5MPH at 50MPH speeds.


Let's hope they regulate the flow at gasoline pumps at tighter tolerance than +/- 10%.

Oh wait.... Don't care.


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## Needsdecaf (Dec 27, 2018)

Margin of error pretty small. Somewhere between 81 and 82 indicated is 80 actual per Waze and another speedometer app on my phone. Meaning if I set to 81, Waze will vascilate between 79 and 80, and if I set to 82, it will between 80 and 81.


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## brur (Nov 15, 2018)

I know my observation means nothing in the big world, but here it is: The meter seems to read one mile an hour, maybe two, slow no matter how fast I go.
This is based on those digital readouts beside the highway. The fact that every one of them reads the same is why I put any semblance of belief in them. Still, they might all be calibrated by the same faulty tech.


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## Frully (Aug 30, 2018)

Keep in mind a lot of this will have to do with tire wear, temperature, and by extension inflation pressure.
A few psi one way or the other changes the effective radius of the wheel, changing the distance traveled by the rotation sensor. Yes, it would make sense to ratify that against the gps calculated speed to account for wear (which it may very well do)...
fwiw Mind tends to read 1km/hr faster than I'm doing. If I cruise 30km/h the radar sign reads 29. 

stock 19" rubber radius is 235mm, with 6mm of treadwear life. 
fresh pi*D = 470mm * 3.142 = 1477mm circumference
worn = 1439mm
a 2.5% difference just in treadwear. 

Inflation and rotation speed play a nontrivial role in maintaining the circular shape of the tread which plays a non-linear role in maintaining velocity/rpm magic math.


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## PNWmisty (Aug 19, 2017)

Frully said:


> fwiw Mind tends to read 1km/hr faster than I'm doing. If I cruise 30km/h the radar sign reads 29.


Keep in mind that the roadside radars will always read a bit slower than actual due to cosine error. If you're not familiar with cosine error this is a good place to learn about it:

https://copradar.com/chapts/chapt2/ch2d1.html

It even has a cosine error calculator so you can see exactly how much error a particular roadside radar has.

A GPS can be a very accurate way to measure speedometer accuracy but not all GPS's are good for this. I've used an aviation rated Garmin with an external roof-mounted antenna on other vehicles with good results. The road needs to be straight and the speed needs to be held constant for a few seconds because the GPS averages the speed over time. I doubt the phone apps are good enough to get within better than 1 mph or so.


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## M3OC Rules (Nov 18, 2016)

android04 said:


> I hope the new map data has updated speed limit info. Most highway speeds in Nebraska increased by 5 MPH about a year ago and the Tesla data still has the old speeds. So it sucks using Autopilot on restricted roads because it only allows 5 MPH above what it thinks is the limit. Resulting in only being able to go about 3 MPH below the actual limit since the Model 3 reads about 3 MPH higher than reality.


The speed limit data comes from Google. You can inform them of errors and they may fix them but I've tried and failed. There is also no way to tell what Google has in their database for speed from their maps so it's not easy. The road is broken up into sections which complicates things even further. The instructions are: Send Feedback=>Report a data problem=>Tap on the road where the Speed Limit data is wrong=>Add a note that the speed limit is wrong=> send your report.

I think this will continue to be a problem until Tesla moves to a different solution. This issue definitely restricts my autopilot usage. They should be reading signs pretty soon for FSD but it's not clear if they are going to do that on HW2.5 or on regular autopilot. Reading signs will help but you need some smarts beyond just reading signs.


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## MelindaV (Apr 2, 2016)

M3OC Rules said:


> The speed limit data comes from Google. You can inform them of errors and they may fix them but I've tried and failed. There is also no way to tell what Google has in their database for speed from their maps so it's not easy.


You can see the posted speed in google maps when using their mobile apps for navigation.

I have a section of a 50MPH freeway that shows up as 70MPH for about 3/4mile. I reported it to google, later confirmed it doesn't show the error in the google maps navigation app (iOS), but Tesla's info still lists it as 70, even after the navigation update last month (correction showed up with Google earlier this year). So not convinced that data is coming from Google.


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## M3OC Rules (Nov 18, 2016)

MelindaV said:


> You can see the posted speed in google maps when using their mobile apps for navigation.
> 
> I have a section of a 50MPH freeway that shows up as 70MPH for about 3/4mile. I reported it to google, later confirmed it doesn't show the error in the google maps navigation app (iOS), but Tesla's info still lists it as 70, even after the navigation update last month (correction showed up with Google earlier this year). So not convinced that data is coming from Google.


True that's good for verifying the error. But you can't send in an error report while driving very easily. When you go to report it you have to remember exactly where the error is. PITA. I checked one place and Google's error aligned with Tesla. Maybe it's out of sync? It seems like Tesla should be upfront about where the data comes from so people can help fix it. Or better yet come up with a better solution.


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## android04 (Sep 20, 2017)

I also agree that the speed limit info doesn't come from Google. I've fixed the speed limits in Open Street Map, Google Maps, and Tom Tom. This latest maps update still doesn't have my local speed limits fixed. It might still be Tom Tom since I had that one fixed right before the update. But it's such a pain to update segments of road in Tom Tom's editor.


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## M3OC Rules (Nov 18, 2016)

MelindaV said:


> So not convinced that data is coming from Google.





android04 said:


> I also agree that the speed limit info doesn't come from Google. I've fixed the speed limits in Open Street Map, Google Maps, and Tom Tom. This latest maps update still doesn't have my local speed limits fixed. It might still be Tom Tom since I had that one fixed right before the update. But it's such a pain to update segments of road in Tom Tom's editor.


I double-checked a section I drive on my commute. It's a long entrance ramp. The speed limit is 55mph. They both show this. Then at the exact same spot the speed limit goes away on Google Maps while the Tesla switches to 30mph. This goes for quite a while, even a little on the freeway as its a new lane, and finally both switch to 60mph. So they have the exact same error. I suppose they could get the data from another common source.

Needless to say, entering a freeway at 30mph is not cool.


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## Mr. Spacely (Feb 28, 2019)

M3OC Rules said:


> The speed limit data comes from Google. You can inform them of errors and they may fix them but I've tried and failed. There is also no way to tell what Google has in their database for speed from their maps so it's not easy. This issue definitely restricts my autopilot usage. They should be reading signs pretty soon for FSD but it's not clear if they are going to do that on HW2.5 or on regular autopilot. Reading signs will help but you need some smarts beyond just reading signs.


It's funny that the first 12 posts completely missed the point of the OP-- posted speed limits vs. what the car is suggesting is the correct speed limit. Not "my car is off by 1 mph because I have 19" tires..."


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## M3OC Rules (Nov 18, 2016)

Mr. Spacely said:


> It's funny that the first 12 posts completely missed the point of the OP-- posted speed limits vs. what the car is suggesting is the correct speed limit. Not "my car is off by 1 mph because I have 19" tires..."


To be fair they did mention that the speedometer being off exacerbates the issue but that and the title did create two threads in one.


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