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Speedometer: Inaccurate?

ElSupreme

Model S 03182
Jan 13, 2012
4,303
92
Atlanta, GA
Those side of the road speed signs are rarely calibrated. And most of the time very poorly setup. They are not reliable measures of real speed.

The best way to check is with a GPS device, or use a GPS app on your phone.

I have only seen 1mph different using this method. And normally when gaining/losing elevation, which would account for the difference. I think the Model S (at least mine) has a pretty dead on speedo. Although I haven't done any rigorous testing this has been my casual observations.
 

Raffy.Roma

Active Member
Jul 22, 2012
3,275
23
Rome (Italy)
@brianstorms

It's normal for speedometers of any kind (also digital speedometers) to have a tolerance of some percentage. In your case the tolerance of the speedometer looks to be 1%. It's good. it's not a matter of calibration IMO, but of accuracy of the measuring device (speedometer) which is in the normal range of all speedometers (tolerance of 1%).
 

Denarius

Active Member
Apr 27, 2013
1,378
84
Utah
All of my other cars have been off by about 4-5mph at 75. My Model S is off by 1mph. Check it with GPS. Radar has to be calibrated for the atmospheric pressure, only laser is accurate all the time.
 

Doug_G

Lead Moderator
Apr 2, 2010
17,881
3,351
Ottawa, Canada
FWIW, when I drive past similar fixed radars the reading on my speedometer is always identical.

This. They put one of these gadgets up in my neighbourhood periodically, and it always matches within 1 kph.

I did a charity drag race event this summer where they used a radar gun to measure top speed. They measured 203 kph. My speedometer reported 203 kph (I have it on video!).

The Model S speedometer is by far the most accurate I have ever seen.

Could easily be poor calibration of the roadside sign gizmo. Also bear in mind that if the radar is well off to one side, it will get a cosine error and report a speed lower than actual.
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,705
22,701
Texas
On mine the speedometer shows 1 mph higher than the curbside radar. I've been through a number of different ones in the various cities around here, and they all read the same.
 

Red

Member
Sep 27, 2013
34
0
earth
Actually, GPS and actual radar will always show about 7% lower speed than your speedo on any certified production car becuase your tacho is supposed to lie to you by that much, to keep you on the safe side of the law. Add to that the tolerance of speed traps and you can actually speed over the limit by some 10-15% safely. Not suggesting it, just saying.

This is normal. Your tacho is OK when it shows those 7% over actual speed. In any car.
 

Doug_G

Lead Moderator
Apr 2, 2010
17,881
3,351
Ottawa, Canada
Actually, GPS and actual radar will always show about 7% lower speed than your speedo on any certified production car becuase your tacho is supposed to lie to you by that much, to keep you on the safe side of the law. Add to that the tolerance of speed traps and you can actually speed over the limit by some 10-15% safely. Not suggesting it, just saying.

This is normal. Your tacho is OK when it shows those 7% over actual speed. In any car.

Except Model S. The speedometer is accurate.

The Roadster is a different story. The main speedometer is very inaccurate, and actually not consistent, but the apparently GPS-calibrated speedometer on the touchscreen is always spot on.
 

ZBB

Emperor
Feb 27, 2013
1,549
270
Scottsdale
Actually, GPS and actual radar will always show about 7% lower speed than your speedo on any certified production car becuase your tacho is supposed to lie to you by that much, to keep you on the safe side of the law. Add to that the tolerance of speed traps and you can actually speed over the limit by some 10-15% safely. Not suggesting it, just saying.

This is normal. Your tacho is OK when it shows those 7% over actual speed. In any car.

I don't have a direct link to the DOT regulation, but the regulation allows speedometers to be inaccurate on the high side, but cannot report you going a slower than actual speed. i.e. if you are actually going 50, it can display 50, 51, 52, etc. But it cannot display 49. Most manufacturers build in a % error to make sure they are not violating the regulation. In past cars, I've seen 1-4 mph over-reported when going 60 (confirmed by GPS that was stated to be accurate to +/- 0.1 mph). The Tesla appears to be 1 over (although I no longer have that GPS to confirm...).
 

N4HHE

Member
Oct 15, 2013
715
57
Madison, AL
I don't have a direct link to the DOT regulation, but the regulation allows speedometers to be inaccurate on the high side, but cannot report you going a slower than actual speed. i.e. if you are actually going 50, it can display 50, 51, 52, etc. But it cannot display 49. Most manufacturers build in a % error to make sure they are not violating the regulation. In past cars, I've seen 1-4 mph over-reported when going 60 (confirmed by GPS that was stated to be accurate to +/- 0.1 mph). The Tesla appears to be 1 over (although I no longer have that GPS to confirm...).

The DOT spec is +- 5 MPH at 50 MPH. Pretty darn sloppy.

The EU spec is -0% +10%. In Europe a speedometer may not indicate slower than the vehicle is actually moving.

- - - Updated - - -

First the facts:

• Standard 85kWh Model S
• Factory 19" wheels and Michelin tires
• Took delivery Aug 29th.

and now the observations:

• When driving through city or neighborhood streets that have automated speed radar signs that light up and show you your actual speed vs. the posted speed, I find what my S says is my speed is usually 5-10% higher than what these radar signs say.

For example, in a neighborhood with 25mph radar signs, I'll go through with the speedometer saying 25mph exactly, and I notice the radar speed limit sign doesn't even light up -- suggesting I'm going below 25mph.

But if I go through at say 27mph, the radar speed limit sign says 25.

Likewise, on other roads where the speed limit is 35mph, I see the same thing. If I go 35mph, the sign stays asleep. If I go by it at say 37-38-39 mph (according to my speedometer) the sign will light up and flash between 35 and 36.

Makes me wonder if the speedo in my car is not calibrated correctly? Anyone else seen anything like this?

Basic trigonometry. Your speed on a vector headed to one side of the radar sign will always be less from the perspective of the radar sign than had you been headed straight for it.

If I was designing a radar sign I would make provisions to enter the angle and distance for calibration. But far too often those erecting such signs don't understand such things and wouldn't make the calibration even if it was available.
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,705
22,701
Texas
The DOT spec is +- 5 MPH at 50 MPH. Pretty darn sloppy.
I'm pretty sure the DOT regulations date from bias-ply tire days. Bias-ply tires can easily vary their RPMs by 30% just from inflation differences. A ten MPH range is actually quite tight given that. Of course it would have been better to update the regulations but I doubt there have been many complaints.
 

ken830

Model S 85, Model 3 Performance
Jun 19, 2012
1,111
365
San Carlos, CA
Car and Driver did an article on speedometer accuracy more than a decade ago...

http://www.caranddriver.com/features/speedometer-scandal

Basically... many cars have speedometer reads faster than reality. In general, European cars are the most exaggerated followed by Japanese Cars. The most accurate are American cars.

Sorted by price, luxury cars are the least accurate, and cars costing less than $20,000 are the most accurate. By category, sports cars indicate higher speeds than sedans or trucks. Cars built in Europe exaggerate more than Japanese cars, which in turn fib more than North American ones. And by manufacturer, GM's domestic products are the most accurate, and BMW's are the least accurate by far. One other trend: Only 13 of our 200 test speedos registered below true 70 mph, and only three of those were below 69 mph, while 90 vehicles indicated higher than 71 mph.
 

neroden

Model S Owner and Frustrated Tesla Fan
Apr 25, 2011
14,676
62,627
Ithaca, NY, USA
First the facts:

• Standard 85kWh Model S
• Factory 19" wheels and Michelin tires
• Took delivery Aug 29th.

and now the observations:

• When driving through city or neighborhood streets that have automated speed radar signs that light up and show you your actual speed vs. the posted speed, I find what my S says is my speed is usually 5-10% higher than what these radar signs say.

For example, in a neighborhood with 25mph radar signs, I'll go through with the speedometer saying 25mph exactly, and I notice the radar speed limit sign doesn't even light up -- suggesting I'm going below 25mph.

But if I go through at say 27mph, the radar speed limit sign says 25.

Likewise, on other roads where the speed limit is 35mph, I see the same thing. If I go 35mph, the sign stays asleep. If I go by it at say 37-38-39 mph (according to my speedometer) the sign will light up and flash between 35 and 36.
It's your local radar signs which are inaccurate. We have similar radar signs in Ithaca and they report exactly the same numbers as my speedometer.
 

ZBB

Emperor
Feb 27, 2013
1,549
270
Scottsdale
The DOT spec is +- 5 MPH at 50 MPH. Pretty darn sloppy.

The EU spec is -0% +10%. In Europe a speedometer may not indicate slower than the vehicle is actually moving.

thanks for the correction... It must have been the EU reg I was thinking of...
 

Musterion

18h 03m 37s −24° 23′ 12″
Jan 10, 2013
579
168
M8
Does the speedo readout from the streaming data in the phone app when tracking the car use GPS or the car speedo as its source? If GPS you could start there to compare.
 

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