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Via word-of-mouth, I have twice heard claims that speedometers are deliberately designed to read high, so manufacturers would not be subject to litigation for speeding.

When I first heard this claim, I thought it was simply an excuse to justify speeding. However, I've noticed that this claim is even on Wikipedia (ref.), albeit unreferenced:

Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits.

So my question is, simply, is this true?

Question: Do manufacturers deliberately calibrate speedometers to read high?

There seems to be plenty of such claims online, but I didn't find any trustworthy evidence to support the claim.

Moreover, this claim seems inconsistent with government-enforced accuracies on speedometers (i.e., if speedometers were deliberately designed to be incorrect, then they would be less likely to meet government standards).

Oddthinking
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Douglas S. Stones
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    Tire wear springs to mind first, a not insignificant bias. On a tire with 25 inch overall diameter, 1/8 inch of tread loss means a 1/4 inch change in diameter. This means @50 mph, your speedo will predict you are traveling at 50.5 mph even for a small loss of tread. Drive on bald tires and the bias may be several times that large. –  Aug 08 '12 at 11:35
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    But I think more important is the "weatherman" bias. (The weatherman tends to predict rain if there is even any chance of rain. Thus people may be happy when it is supposed to rain but does not. But if you predict it will be nice, and a tornado comes though, they are rarely happy. So weather forecasts tend to be biased.) You prefer the speedo predicts high, which lessens the chance of a ticket if it is in error. The manufacturer also should prefer errors on the high side, as that lessens the odds of any potential lawsuit. –  Aug 08 '12 at 11:46
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    Another good point is, ALL measurement devices will be in error. Errors will often naturally follow a Gaussian-like distribution, thus having tails on either side. But suppose you simply remove all devices that predict low? Now you have a skewed distribution, created not by design, but by restriction where the error is biased to always predict high. Even if you remove only those devices that are below -1xsigma, this still skews the distribution to be biased high. –  Aug 08 '12 at 11:55
  • Here is a source that claims actual speed vs shown speed : http://click2how.com/your-speedometer-wrong-can-drive-faster/ - If true that shows up to a 10% margin of error meaning your warranty will wear out 10% earlier than it actually should since the odometer goes off the speedometer. – Chad Aug 08 '12 at 16:06
  • @Chad, any sources that the odometer goes off the speedometer? – ardent Aug 08 '12 at 16:56
  • @Ardentsonata - That is how US Standard Speedometers work. I can not say that is true for non US... though I would be surprised if it did not. – Chad Aug 08 '12 at 18:33
  • @Chad, that seems dubiously legal for an odometer to be based off a speedometer that isn't necessarily accurate. Or at the very least, incredibly deceptive in regards to gas mileage, warranties, and resale value. – ardent Aug 08 '12 at 18:41
  • @ardentsonata - Well the EPA made them change how they did the milage reporting for sales and it always seems below what my computer says it is... so if that is the case then yes I am feeling ripped off. Although I will say that there have been a few times that I was caught on radar doing the exact speed that my car said i was doing... its a conspiracy at the highest levels! – Chad Aug 08 '12 at 18:42
  • @Chad, well then color me even more cynical about the auto-industry. Guess this is a good thing to know. CAN'T TRUST ANYTHING THESE DAYS. ALL OF DEM CONSPIRACIES. – ardent Aug 08 '12 at 18:44
  • As an aside... I went to the how stuff works site looking for a reference for you and they said the scene from Ferris Beuler should have worked... they fixed that problem in the 50's so cant take that site for reference any more. – Chad Aug 08 '12 at 18:50
  • http://www.essortment.com/auto-questions-odometers-work-57556.html The key quote "it also shares the same flexible cable if they run mechanically." – Chad Aug 08 '12 at 18:54
  • let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/4430/discussion-between-ardentsonata-and-chad) – ardent Aug 08 '12 at 20:47
  • Both answers here are good and accurate, but would be better if their first word was "Yes: ", since the devices are virtually guaranteed to always err and to err to the side of higher speeds. – Ofir Sep 01 '16 at 08:56

2 Answers2

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In the EU, speedometers are not allowed to display a speed that is lower than the speed the vehicle is traveling. Manufacturers calibrate speedometers to comply with this regulation as they would not be allowed to sell their cars if they did not.

Reference - EU Regulations for Speedometers (75/443/EEC)

The speed indicated must never be less than the true speed. At the speeds specified for the test in 4.3.5 above and between these speeds, there shall be the following relationship between the speed indicated on the dial of the speedometer (V1) and the true speed (V2):

formula

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Tom77
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    The US DOT has a similar requirement [http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?reg=393.82] that the measured value @ 50mph be within +/- 5mph (80km/h +/- 8km/h). If I were a speedometer manufacturer I know which way I'd calibrate my errors... – KutuluMike Aug 08 '12 at 14:42
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    Speedo manufacturers could be held liable if the dial read low and you, trusting it, were caught speeding, so because there will always be some error, the manufacturers calibrate to make the lower error bar higher than true speed. – Rory Alsop Aug 08 '12 at 14:49
  • Thanks! That's pretty solid evidence to show the claim is true in the EU. Although it leaves open the question as to what happens outside the EU. – Douglas S. Stones Aug 08 '12 at 22:46
  • The only evidence i have is that i recently emigrated from europe to outside europe. My car used to read about 4mph too fast, now it reads about 5kph too fast. So i suspect the same situation exists. – Sirex Aug 09 '12 at 04:12
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I don't think speedometers are deliberately set to read high, but you are less likely to get a speeding ticket that way than if they read too low. If they read high, you can guarantee to be below the speed limit, even when your speedometer indicates you're slightly over.

EDIT The first sentence above conflicts with what follows below: if the manufacturer is not permitted to apply a speedo that reads low, that can only be done by making sure it reads high. The only other possibility is to make it read exactly right, which is obviously impossible.

In Australia the indicated speed must not be lower, but can be up to 10% + 4km/hr higher than the actual speed. This applies when the car is tested as prescribed (correctly inflated standard tyres, unloaded, etc). The testing procedure and accuracy specifications are here (Motor Vehicle standards act 1989, Australian design rule 18/03 (.pdf)):

5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5. above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).

0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h

Because of this, police in Victoria only let you go about 2km/hr over the speed limit before they book you. As far as I know, New South Wales still allows you 10 km/hr.

More info about speeding laws in Australia:

http://www.trafficlaw.com.au/speedos.html

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hdhondt
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  • This answer (along with Tom77's answer) is pretty clear-cut. I'll accept this one, since I'm Australian. Although, the choice was mostly arbitrary. – Douglas S. Stones Sep 10 '12 at 22:33