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Speedometer off by -1 MPH

mmh

Member
May 11, 2014
122
0
SLC
Curious if anyone else has noticed the speedometer being off by -1 MPH, e.g. car says 60MPH but in reality is going 59MPH. I can verify this with both GPS and those radar speed limit sign things.

6046_61MPH_CLSTuPg.JPG
 

astrotoy

Supporting Member
Jan 24, 2013
321
674
SF Bay Area
Mine is exactly on, when it is stopped!

On my 2007 Lexus LS460L the car reads about 3 mph fast for speeds above about 20 mph. In the Lexus group there was talk about challenging Lexus about their warranty, since it is both time and mileage based and the odometer shortchanges the warranty on mileage. No issue with unlimited Tesla mileage.
 
Last edited:

bollar

Disgruntled Member
May 1, 2013
2,667
878
Southlake, TX
Unless the car gets its speed from GPS, the speed displayed comes from wheel revolutions and is subject to change as the tires' circumference reduces due to tread wear.
 

Johann Koeber

Active Member
May 1, 2012
1,179
3,392
Hersbruck, Germany
Well - according to German law the speedometer can only have a tolerance to the high side. It can never show a speed lower than the one you are driving.

The formula is

actual speed < speed shown by speedometer < actual speed + 10 % plus 4 km/h

Source:
§ 57 Par. 2 StVO
Anlage zu § 57 Par. 2 StVO

I assume there are similar laws in other countries.

The MS has the most accurate speedometer of any car I have so far driven - I love it. Until I knew this, I had to pay one (small) speeding ticket. But now I know!
 

AnOutsider

S532 # XS27
Apr 3, 2009
11,957
200
I've noticed a small discrepancy, but never really tested it to get an accurate number.

All of our previous cars (German) were off as well, and it scaled as you went faster). There was "talk" on some old forums about German automakers doing this because they didn't want to be held liable if a ticket was issued saying they were going faster than the speedo indicated.

Don't know how true that is, but almost immediately I noticed the Model S always seemed to indicate a speed closer to real speed than past cars.

*edit* and it seems Johann ninja'd me and confirmed the chatter.
 

ItsNotAboutTheMoney

Well-Known Member
Jul 12, 2012
10,420
7,548
Maine
Well - according to German law the speedometer can only have a tolerance to the high side. It can never show a speed lower than the one you are driving.

The formula is

actual speed < speed shown by speedometer < actual speed + 10 % plus 4 km/h

Source:
§ 57 Par. 2 StVO
Anlage zu § 57 Par. 2 StVO

I assume there are similar laws in other countries.

US law is similar.

PS The Prius is a big fat liar on speed and fuel economy.
 

Tommy

Member
Mar 3, 2010
882
3
The great OC
Not a scientific measurement, however I have driven by many portable radar speed display trailers and my car's speed always shows at least one mile higher on the display than what the speed sign shows.

My other cars also show I am traveling faster than what the radar displays show, so it's not just Tesla that does this. Personally, probably a good thing due to the ease at which the S gets up to speed (and beyond).
 

mmh

Member
May 11, 2014
122
0
SLC
I just want to add; The Tesla has the most accurate speedometer of any car I've ever driven. I am just curious if anyone else noticed the speeds consistently off by 1MPH. I'm trying to determine if it's a margin-of-error thing, or if it's purposely wrong by Tesla.

//On a side note, I have a BMW K1200S motorcycle which has always been dead accurate.
 

Paul Carter

Active Member
Apr 27, 2013
1,735
510
Canada
I notice my car has a 2km faster reading than the speed zone radar signs which are around here. So that's just above 1 mile, so seeing the same thing here.

So now what, I do some fancy math for cruise control... speed limit + 10% + 2km.
 

AudubonB

One can NOT induce accuracy with precision!
Mar 24, 2013
8,098
27,023
Let's work through the logic here:

IF

Speedometer off by -1 MPH

THEN is the following also true?:

Speedometer on by +1 MPH

???!!!:biggrin:
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,632
22,279
Texas
The roadside radar almost always shows +1 mph, the few times when it doesn't show +1 it's zero.
 

Vger

Active Member
Apr 10, 2009
1,781
209
Salt Spring Island, BC, Canada
The roadside radar almost always shows +1 mph, the few times when it doesn't show +1 it's zero.

This is my experience as well. I was just watching this during a long road trip. Given the Elon design ethic, I would imagine that they calibrate the speedometer to be 0.5 mph high, just to be legal, but to be otherwise as accurate as possible!

I really appreciate being able to rely on the speedometer as accurate, as well as the incredible stability of the cruise control!
 

green1

Active Member
Mar 25, 2014
4,548
1,121
Calgary, Alberta, Canada
Speedometers on all cars read high from the factory. Modern cars less so than older ones as accuracy of the components is easier to guarantee now, but always high. It gives the manufacturer some wiggle room so as not to break the law stating they can't read low. (It also saves them from owner lawsuits when someone gets a speeding ticket!)
 

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