Speedometer appears to always over-read actual speed

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Mike

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#1
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)

--------------------------------------------------------------------------------------------------------------------------------------------

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....:confused:
 

joelliot

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#2
...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.
 

Mike

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#3
...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.
 

garsh

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#4
...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.
 

garsh

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#6
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. ;)
 

Wilson

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#7
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.
 

SoFlaModel3

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#8
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)

--------------------------------------------------------------------------------------------------------------------------------------------

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....:confused:
So you’re telling me I can drive MOAR FASTERER without getting in trouble? ;)
 

Mike

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#9
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?
 

JWardell

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#10
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!
 

racekarl

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#11
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.
 

PNWmisty

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#12
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.
 
Last edited:
4

4701

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#14
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".
 

PNWmisty

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#15
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?
 

ADK46

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#17
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.
 

Eigenv1

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#18
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)

--------------------------------------------------------------------------------------------------------------------------------------------

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....:confused:
 

PNWmisty

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#20
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....:confused:
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.