# Speedometer appears to always over-read actual speed



## Mike (Apr 4, 2016)

I figure I would start this thread to generate some data points.

The following is copy and pasted from a comment I made inFirmware Build 2018.26.3 be4b11e (7/27/18)

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Disclaimer: This was one test, using only one (1) stand alone GPS unit plugged into my car and mounted to the windscreen.

All figures in kilometers.

Test took place at an approximate elevation of 90 meters.

Temperature throughout the test was 27C indicated.

All figures were repeatable at least three (3) times on this test loop.

Car indicates: GPS indicates:

45............... 44
50............... 49
60............... 59
70............... 68.5
80..................... 78.5
85............... 83.25
90............... 88
100............... 98
105............... 103
110............... 108
115............... 113
120............... 117.5

Trip Odometer:

Car: 45.9..... GPS: 45.6

Conclusions:

The car's speedometer indicates 2% higher speed than the GPS.

The car's odometer over-indicates distance by a factor of 1.0066.

FWIW: 80,000 indicated would equal 79,475 (a delta of 525).

192,000 indicated would equal 190,741 (a delta of 1259).

Now I know why my dead reckoning math never seems to work when comparing my mental ETA with the car nav system ETA....


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## joelliot (Jan 25, 2018)

...not an expert on this, but I would think a couple of percent was normal for new tires. If someone knows the useable thread depth, we could probably calculate where you would be with worn tires.


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## Mike (Apr 4, 2016)

joelliot said:


> ...not an expert on this, but I would think a couple of percent was normal for new tires. If someone knows the useable thread depth, we could probably calculate where you would be with worn tires.


My tires have 6400 kms of wear.

Visually, I consider the tread to not be worn because the wear bars do not stand out.

I'm going to assume the tires are still not worn to cause a 2% speedometer variance.

I'd be interested if other folks have the same or similar results.


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## garsh (Apr 4, 2016)

joelliot said:


> ...not an expert on this, but I would think a couple of percent was normal for new tires. If someone knows the useable thread depth, we could probably calculate where you would be with worn tires.


As a tire wears, the car will report going even faster than the actual speed.


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## joelliot (Jan 25, 2018)

garsh said:


> As a tire wears, the car will report going even faster than the actual speed.


Yes, sorry had this exactly backwards. Smaller tiers will have to spin faster. ...long day


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## garsh (Apr 4, 2016)

joelliot said:


> Yes, sorry had this exactly backwards. Smaller tiers will have to spin faster. ...long day


No worries. I had to think it through twice before I felt confident enough to write that post.


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## Wilson (Jun 2, 2017)

I read an article a few years ago that said most cars show one to two mph faster than actual. The article stated that this was true for multiple manufacturers and countries of origin. Comparing with the gps speed readout on Waze, 2 out of 3 of my past vehicles have confirmed this. A Boxster and a Touareg showed 1 to 2 mph fast and my Golf shows the same speed as Waze.


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## SoFlaModel3 (Apr 15, 2017)

Mike said:


> I figure I would start this thread to generate some data points.
> 
> The following is copy and pasted from a comment I made inFirmware Build 2018.26.3 be4b11e (7/27/18)
> 
> ...


So you're telling me I can drive MOAR FASTERER without getting in trouble?


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## Mike (Apr 4, 2016)

Wilson said:


> I read an article a few years ago that said most cars show one to two mph faster than actual. The article stated that this was true for multiple manufacturers and countries of origin. Comparing with the gps speed readout on Waze, 2 out of 3 of my past vehicles have confirmed this. A Boxster and a Touareg showed 1 to 2 mph fast and my Golf shows the same speed as Waze.


I've heard that as well.

If it proves to be systemic to the Model 3, why not have a software update to include the speedometer being corrected?


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## JWardell (May 9, 2016)

This is very common, and speedometers intentionally read higher for liability.
Often if you have an OBD reader you can read the car's calculated speed, below what it's showing on the speedo.
I Used to use a ScanGuage II mounted on my dash as a speedometer because it lets you apply a correction factor...my old mini was over 6% fast!


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## racekarl (Jul 31, 2018)

This is generally true of all cars. In the EU, regulations specify that a speedometer cannot read below the actual speed, so manufacturers make sure theirs read high from the factory to leave themselves a margin for error for different wheel/tire combinations, etc.


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## PNWmisty (Aug 19, 2017)

Mike said:


> My tires have 6400 kms of wear.


When doing this type of testing, it's also necessary to specify the tire pressure (and brand/model/size if different from OEM). After warming up the tires the pressure will increase which increases the diameter and decreases the calculated speed. Which is why precision distance measuring devices always use tires that are non-pneumatic.

In the US, regulations allow for quite a bit of leeway on over-reporting the speed, but zero tolerance for underreporting. It looks like Tesla is more willing to push it closer to the line of accuracy than many other makers.


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## Mesprit87 (Oct 29, 2017)

I have noted the same as above with all my cars. The thing to remember is that the mileage you are doing is precise.
They are messing with your speed but not with your warranty


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## Guest (Aug 17, 2018)

Car might know the actual speed. *Actual speed* (GPS), *calculated speed *and* displayed speed* can all be different.
To observe actual measured speed, set cruise to fixed value (let's say 50). Reset average speed. Wait a minute. Observe
"average speed since reset".


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## PNWmisty (Aug 19, 2017)

arnis said:


> Car might know the actual speed. *Actual speed* (GPS), *calculated speed *and* displayed speed* can all be different.
> To observe actual measured speed, set cruise to fixed value (let's say 50). Reset average speed. Wait a minute. Observe
> "average speed since reset".


Do you care to share the results you got when you tried this?


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## Guest (Aug 17, 2018)

I have a different vehicle (two actually). One is 10%+3km/h off, another is 0%+4km/h off.
No Model3, yet.

PS: it also matters what tires. Winter tires are slightly larger, even with same dimensions.


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## ADK46 (Aug 4, 2018)

The numbers in the table show an excellent correspondence, just what you’d want - a little high. I have a new Porsche that reads 3-4 mph high, which I don’t think is so good.

Then again, a “perfect” readout is now feasible by several means, such as constant calibration of the wheel-based (or motor-based) speed from GPS readings (not perfect moment by moment, but perfect over longer periods on straight roads). Or the same sort of laser business that computer mice use.


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## Eigenv1 (May 8, 2017)

Mike said:


> I figure I would start this thread to generate some data points.
> 
> The following is copy and pasted from a comment I made inFirmware Build 2018.26.3 be4b11e (7/27/18)
> 
> ...


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## Eigenv1 (May 8, 2017)

It's Tesla's way of helping you, in a very small way, of avoiding speeding tickets.


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## PNWmisty (Aug 19, 2017)

Mike said:


> Trip Odometer:
> 
> Car: 45.9..... GPS: 45.6
> 
> ...


I think your trip odometer is probably more accurate than would be indicated by doing a "test loop" because a GPS calculates distance using a series of straight line segments to represent a curve while a car actually has to go all the way around the curve. To get a more accurate odometer calibration, repeat the test on a long, straight road.


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## Mike (Apr 4, 2016)

PNWmisty said:


> I think your trip odometer is probably more accurate than would be indicated by doing a "test loop" because a GPS calculates distance using a series of straight line segments to represent a curve while a car actually has to go all the way around the curve. To get a more accurate odometer calibration, repeat the test on a long, straight road.


The majority of that one test was done on a straight section of Hwy 401 near my place.

Based on my drive down to southern New Jersey and back last week, using the mile markers every 1/10th of a mile as found on sections of I476, my odometer indeed has a "bias" to read faster than the mile markers posted on that long stretch of toll-road.

If you only travel 10 miles, you may not notice it.

But hack (to zero) the trip odometer when you pass one of the mile markers and monitor it over the next 100 miles.....the trip odometer and the highway mile markers slowly go out of sync and _always_ with the car reading faster.


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## PNWmisty (Aug 19, 2017)

Mike said:


> The majority of that one test was done on a straight section of Hwy 401 near my place.
> 
> Based on my drive down to southern New Jersey and back last week, using the mile markers every 1/10th of a mile as found on sections of I476, my odometer indeed has a "bias" to read faster than the mile markers posted on that long stretch of toll-road.
> 
> ...


That's essentially what I'm saying. Odometer accuracy can be measured with accurate mile markers on a straight road or a GPS on a straight road but not by using cumulative GPS readings from a "test loop" due to inaccuracies introduced by the way in which GPS's turn curves into a series of straight segments.


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## metroplex (6 mo ago)

My 2022 M3 Long Range has this same issue. Car's indicated speed on the freeway is 70 mph, GPS reading is 69 mph. I know something was up because my Uniden R7's GPS speed reading is always higher than normal and when it matched the Tesla's speedometer, I had to double check with another GPS.

Is there a way to recalibrate the speedometer? On my Fords, I'd go in and change the tire circumference using FORScan or HPTuners.


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## garsh (Apr 4, 2016)

metroplex said:


> Is there a way to recalibrate the speedometer?


I bought tires with a slightly larger diameter for winter (245/45R18). That made the speedometer match GPS, at least until the tires wore down a bit.


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## SoFlaModel3 (Apr 15, 2017)

This is not uncommon for cars to read wrong in either direction right from the factor. Not that it makes it right, but again common.


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## RickO2018 (Mar 13, 2018)

Mike said:


> I figure I would start this thread to generate some data points.
> 
> The following is copy and pasted from a comment I made inFirmware Build 2018.26.3 be4b11e (7/27/18)
> 
> ...


I’ve noticed 1 mph over the actual speed as displayed by stationary radar reminders where they are used. As someone already pointed out, I see it as a slight advantage over a potential speeding ticket from an active police radar. What concerns me more is the car’s inability to read and react to speed limits under 25mph.


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## metroplex (6 mo ago)

I don't care about stationary radar readings. I do however want to calibrate the Tesla speedometer to match my 10 Hz aviation grade GPS. This is easily easily easily done on a Ford made between 2005 and present. Less than 30 seconds. On a Tesla it doesn't seem possible


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