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Car Speedometer Accuracy vs GPS: Understanding the 5 km/h Difference

gerta21w2 21027 43
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How accurate is a car speedometer compared with GPS, and is a 5 km/h difference normal?

A 5 km/h difference is usually normal because car speedometers are required to never show less than the actual speed and are allowed to overread by a margin, not display the exact speed. The EU rule quoted in the thread says the indicated speed must satisfy `0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h`, so the error can grow with speed [#21348782] [#21348931] Tyre wear also makes the reading higher than the real speed because the tyre circumference becomes smaller, and changing wheel/tyre size from the factory setup changes the result too [#21348771] [#21348969] GPS is not a perfect reference either: its speed depends on sampling/refresh rate, satellite reception, road shape, and surroundings, so brief differences are expected [#21348811] [#21348931] If you want to check your speedometer without special equipment, compare it with roadside speed display boards or read vehicle speed from an OBD scanner and compare that with the dash reading [#21348775] [#21349059]
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  • #1 21348757
    gerta21w2
    Level 1  
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    Lately, I’ve been thinking about something I’ve noticed while driving: how reliable is the speedometer in my car? I was on a drive recently, and I had my phone mounted with a navigation app that shows speed based on GPS. I noticed a small difference between the speed shown on my car’s speedometer and the speed displayed on the app—around 5 km/h or so. It made me wonder, can we fully trust our car's speedometer, or is this kind of variation normal?

    A speedometer is a tool that measures and displays the speed of a vehicle in real-time. It works by using sensors or mechanical components to calculate the rotation of the wheels and convert this data into a readable value for the driver. These days, tools like an interactive live speed meter are becoming more common in some vehicles, integrating GPS and other advanced features for real-time accuracy. Still, it got me thinking—how accurate is the average speedometer in the cars most of us drive?

    I’ve read that speedometers are designed to slightly overestimate speed as a precaution. If that’s true, it could explain the difference I noticed. But is there a way for regular drivers to verify the accuracy of this tool without needing expensive equipment or relying on apps?

    Have you ever experienced something like this with your speedometer? Do you think this slight difference even matters, or do you trust the tool as it is? And if you’ve ever tried checking your car’s speedometer accuracy, what tools or methods did you use?

    I’d love to hear what others think about this!
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  • #2 21348771
    ArturAVS
    Moderator  
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    Every speedometer has a certain margin of error from the factory, which is due, among other things, to tyre wear (as the tyres wear, their diameter and also their circumference are reduced, this causes the readings to be inflated).
  • #3 21348775
    klm787
    Level 37  
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    gerta21w2 wrote:
    I was wondering if we can fully trust our car's speedometer, or if these kinds of differences are normal?
    .
    Most vehicles underestimate (well that mistake of mine, below a colleague pointed out to me, and it should actually say that overestimates ) the actual speed by a few kilometres.
    And this is actually true:
    gerta21w2 wrote:
    I have read that speedometers are designed to slightly overestimate speed as a precautionary measure.
    .
    And verification (no GPS needed) can be done at such roadside indicators, of which there have been quite a few in recent times.
    They are most often located near pedestrian crossings.
    What is more interesting, they show the speed quite accurately.
    You could even say that they show as much as a GPS
    Roadside radar sign displaying a speed of 27 km/h with a solar panel on top.
    5 km/h is quite a lot.
    On mine it misrepresents by 3 km/h
  • #4 21348782
    szwagros
    Level 33  
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    The EU directive states that the speed shown on the meter must satisfy the formula:
    0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h
    where V2 is the speed actually reached by the vehicle and V1 the speed shown on the clocks.
  • #5 21348784
    blekis
    Level 27  
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    An excerpt from an article:

    "A car sold in the European Union must overstate the speed indicated by the speedometer. This is what the requirements for obtaining EEC approval say. In short, if the car showed the real speed, it would not be approved and therefore could not be sold in the Community. Overstating the speed indicated by the speedometer is therefore a fact. How big are the discrepancies? This is determined by a formula prepared by the EU authorities.

    0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h, where V2 is the speed actually reached by the vehicle and V1 is the speed indicated on the dials.
  • #6 21348786
    milejow

    Level 43  
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    klm787 wrote:
    Most vehicles underestimate actual speed by several
    .
    Overestimates, not underestimates, all speedometers have a measurement error of up to 10%.
    Company Account:
    Działalność własna
    Mętów 99, Lublin, 20-388
  • #7 21348789
    Anonymous
    Level 1  
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  • #8 21348797
    gregor124
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    I think it's logical that if someone is going slower, it's safer than if they were going faster and didn't know it.
    This is regulated by international rules, not just by the EU.
    The general idea is that there should never be a situation where the speedometer underestimates.
    E.g..
    "UK law is based on the EU standard, with some minor variations. The Speedo must never show less than the actual speed and must never show more than 110% of the actual speed + 6.25 miles per hour."
    Helpful post? Buy me a coffee.
  • #9 21348803
    andrzej20001
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    At my place it is 6km/h kia, 5km/h Mitsubishi. Digital meter.
  • #10 21348805
    Aleksander_01
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    blekis wrote:
    This defines the formula prepared by the EU authorities.

    0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h, where V2 is the speed actually reached by the vehicle and V1 is the speed indicated on the clocks"
    .
    Show where you got this formula from, because I have the impression zę "worked it out" yourself, so let's check it out:
    V1=150 km/h is the speed on the clocks
    V2=130 km/h is the actual speed of the vehicle
    0≤20≤13+4 I can see immediately that you developed it yourself in order to shine

    or else - it is true for small differences and even with a perfect reading of the actual speed, the meter will overestimate a minimum of 4 km/h.
    For the rest, every car the further it is from new will overestimate the speed reading.
  • #11 21348811
    Anonymous
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  • #12 21348815
    andrzej20001
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    No basis. I'm not claiming anything. The cars show more and everything. Don't make up theories.
  • #13 21348816
    Aleksander_01
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    Jarzabek666 wrote:
    you think GPS has an accuracy of 100%?
    .
    Exactly.
    I when I'm driving and I'm bored I check the speed from the meter (digital) and from the phone GPS. Very often they coincide, and sometimes it is the case that the car is faster and in a moment the GPS is faster.
  • #14 21348819
    grala1
    VAG group specialist  
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    @Aleksander_01
    According to your data:
    0 ≤ 20 ≤ 17
    Let's take different data:
    V1 = 150
    V2 = 140
    0 ≤ 10 ≤ 18
    That is, the meter cannot overestimate by 20 km/h according to this regulation, but it can by 10 km/h.
    This overestimation of the car meter is not likely to be by a constant value. It is closer to a percentage, but could be something in the formula.

    @klm787, those radars don't show correctly either.
    The ones the police use can't be taken as perfect either.
    Every radar measurement result is subject to an error of greater or lesser magnitude. This is certainly stated in the technical documentation of the device.
  • #15 21348821
    Anonymous
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  • #16 21348824
    andrzej20001
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    And I write what I see on the display of the car,phone,navigation and speed display in my locality. Just this.
  • #18 21348847
    Kiermasz
    Level 23  
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    At my place it is 45-6km/h actual with 50 on the meter.
    And when you go ~55, you magically stop catching all possible red lights :) .
  • #19 21348931
    Gerri
    Mercedes specialist  
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    I have provided the specific Union rule in the PDF attached - indeed the formula for the speedometer error is as stated several times above. It is in section 5.3 of the document.
    The error does not have a constant value - yes, it can be said to have e.g. 5 km/h but at a certain speed . At higher speed this error will be larger, at lower speed it will in turn be smaller. Therefore, for a particular speedometer it will be shown as a percentage of the actual speed of the vehicle.
    My personal impression is that the Union is unnecessarily getting itself into yet another area that nobody invited it into. I have no need to interfere with what the speedometer shows in MY AUC. In my previous car I had the ability to tamper with the meter settings and it eventually showed EXACTLY to the nearest 1km/h the actual speed, in my current car I have a large display on the 2DIN radio with the GPS speed.
    Jarzabek666 wrote:
    do you think GPS has 100% accuracy? GPS is accurate to 5-7M and the speed is calculated from the distance travelled and time. So for a drive of 100m the GPS accuracy is 5%. And that depends on how much it refreshes the GPS speed.
    .
    It's not that simple either. It's not just the refresh rate that affects GPS indications. It is also the type (class) of processor, the shape of the road (straight, winding), the terrain around the road (tall trees, rocks, houses), the weather, the number of satellites received and probably other aspects.
    A large percentage of drivers have no idea about underspeeding, they follow the current readings when driving and here there is already a certain benefit for traffic safety. Many know this, but drive according to the readings anyway. So maybe this is not such a bad thing after all.
    Attachments:
    • liczniki ECE.pdf (154.64 KB) You must be logged in to download this attachment.
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  • #20 21348969
    Lukas6432
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    The meter on the car is scaled to the factory tyres and rims, there are geeks who put on bigger ones and then the meter will show a speed lower than the car is actually going. Larger wheels and a low profile means a larger circumference and a higher speed.
  • #21 21348976
    Gerri
    Mercedes specialist  
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    Lukas6432 wrote:
    Larger wheels and a low profile means larger circumference and higher speed.

    Greater car speed - but on the meter LESS than the actual speed. Then the car can "go out" of the EU model range.
  • #22 21349005
    BANANvanDYK
    Level 42  
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    I have an electronic meter, digital, precisely calibrated to the actual circumference of the tyre when rolling (the manufacturer's quoted ERTO does not reflect reality). It shows perfectly.
    The GPS shows the same amount, but due to the lag and the difficulty of keeping a perfectly constant speed, the accuracy of the readings cannot be compared.
    The radar with speed display also shows the same speed with an accuracy of ±1 km/h, at least at 30 km/h, as I pass one on my way to work.

    >>21348782 .
    This is not a formula taken from the Citroen service manual? Because this formula applies to French, not European law:
    Instruction on speedometers with regulations and examples. .
    I used to have a book, a textbook for a car school, which gave ways to test speedometers (speedometers) at a diagnostic station, but unfortunately the book is lost to me somewhere.
    The question is, which law or regulation in Poland deals with the accuracy of indications and control of speedometers in vehicles? Or the UN ECE regulations?
  • #23 21349059
    ladamaniac
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    If you are bored, I suggest an experiment. Plug in any OBD scanner, find the vehicle speed parameter and compare it with the meter reading. In the old days we used to put on "muzzles", software speed limiters. More often than not, the controller showed the actual speed underestimated against the odometer, although this varied, even within a single make.
  • #24 21350252
    tesla97
    Level 20  
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    Kiermasz wrote:
    It is 45-6km/h actual at 50 on the meter.
    And when you go ~55, you magically stop catching all possible red lights
    .

    It's the same for me, except that when you're going ~70 on the meter it stops catching all possible red lights. For that, I often see that exactly behind me a red light comes on, as if at a moment's notice.
  • #25 21350267
    user64
    Level 35  
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    There is an interesting stretch of road in my area. In a 2km stretch there are 4 junctions with traffic lights, a 50kmh limit, but when you're going 70kmh you magically have green everywhere, when you follow the limits you're standing at a minimum of two, and that's only if you've sprinted to a yellow light before. And I'm talking about 70kmh from the GPS, not the meter.
  • #26 21350338
    James596
    Level 29  
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    No car speedometer shows accurately. In practice I have noticed that the deviation value is not constant. At 100 km/h it is usually around 5 km/h, at speeds around the maximum around 10 km/h.

    Interesting fact - in very old cars the meter can calibrate so that it shows accurately. In my old Ford on nominal tyres, the speeds on the meter coincide with those on the GPS, even at higher readings.
  • #27 21350340
    Mastertech
    Level 30  
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    niewidoczny13 wrote:
    o with me it is already outstandingly different.
    Because going 160, the gps shows me 148km/h
    .
    The truth is: if the speedometer underestimates it, the odometer overestimates it and to cure this you needed to remove the hands from the clocks. Usually the temperature pointer then shows wrong too.
  • #28 21350345
    ArturAVS
    Moderator  
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    Mastertech wrote:
    Unusually the temperature pointer then also shows wrong.

    What does the temperature pointer have to do with speed? Somehow I don't see the connection.

    Mastertech wrote:
    to cure this you would need pictures of the clock hands
    .
    I have not heard a bigger humbug.
  • #29 21350365
    Mastertech
    Level 30  
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    ArturAVS wrote:
    Larger ruckus I've never heard.

    You're up too early, sleep more....
    Others have guessed that I was referring to the electronic counters which had to be taken apart (the clues had to be removed) in order to undo the counter and then when putting it back together it didn't hit right.

    Moderated By gulson:

    3.1.9. Disseminate content that is ironic, mocking or malicious, or which is disrespectful to other Users or third parties.

  • #30 21350373
    ArturAVS
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    @Mastertech Good on you for bragging about breaking the law. I have repaired dozens of meters (repair not mileage correction) and never had this problem.
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Topic summary

✨ The discussion centers around the accuracy of car speedometers compared to GPS readings, highlighting a common discrepancy of about 5 km/h. Participants note that speedometers are designed to overestimate speed due to regulations, particularly in the EU, where the formula 0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h governs acceptable error margins. Factors influencing speedometer accuracy include tire wear, vehicle calibration, and the type of speed measurement technology used (analog vs. digital). Users share personal experiences with various vehicles, indicating that while some speedometers may show a consistent overestimation, others can vary significantly. The conversation also touches on the implications of these discrepancies for driving behavior and traffic regulations.
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FAQ

TL;DR: A 5 km/h gap is usually normal: “overestimates, not underestimates” is how users described most car speedometers. This FAQ helps drivers compare dashboard speed with phone GPS, OBD, and roadside displays, and explains when the difference is harmless and when tyres or wheel changes can make it worse. [#21348786]

Why it matters: A small speed mismatch affects speeding risk, traffic flow, and how confidently you can trust your dashboard in daily driving.

Method Typical reading vs real speed in the thread Main limitation
Car speedometer Often +3 to +6 km/h; sometimes more at higher speeds Built to avoid under-reading
Phone GPS Often closer to true speed on straight, steady runs Lag, refresh rate, satellites, chipset
Roadside speed display Reported as close to GPS or within ±1 km/h at 30 km/h It is still a measuring device with its own error
OBD vehicle speed Often lower than dashboard speed Varies by make and ECU scaling

Key insight: The most important rule is simple: the dashboard speed should not read lower than the car’s real speed. A 5 km/h over-read is common, but tyre size, wear, and wheel changes can increase or reverse the error.

Quick Facts

  • EU-type speedometer over-read was quoted in the thread as 0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h, so at 140 km/h actual speed, a display of 150 km/h still fits the stated limit. [#21348819]
  • Real user examples in the thread ranged from 50 km/h indicated = 45–46 km/h actual to 160 km/h indicated = 148 km/h on GPS, showing that error is not always a fixed value. [#21348847]
  • Several users reported brand-specific offsets around 5–6 km/h on digital dashboards, while another user saw about 3 km/h in their car. [#21348803]
  • Tyre wear reduces tyre diameter and circumference, which raises indicated speed over time even if the sensor system does not change. [#21348771]
  • One user reported a roadside speed display matching their calibrated electronic speedometer and GPS with about ±1 km/h at 30 km/h, but other users warned that roadside displays also have measurement error. [#21349005]

1. Why does my car speedometer show about 5 km/h more than my phone GPS app while driving?

Because most car speedometers are set to read slightly high, not low. The thread repeatedly states that dashboard speed usually overestimates by a few km/h, and several users reported typical gaps of about 3–6 km/h. That design gives a safety margin so the display does not show less than the real speed. A 5 km/h difference is therefore ordinary, especially near urban speeds like 50 or 100 km/h. [#21348775]

2. What does the EU speedometer accuracy formula 0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h actually mean in practice?

It means the speedometer may read higher than real speed, but it must not read lower. In the formula, V1 is indicated speed and V2 is actual vehicle speed. The permitted over-read is up to 10% of actual speed plus 4 km/h. For example, if the car is really doing 140 km/h, the display may legally be as high as 158 km/h under the formula quoted in the thread. [#21348838]

3. How can I check my car's speedometer accuracy without expensive equipment or relying only on a smartphone GPS app?

You can do a simple three-step check with common tools. 1. Drive at a steady speed past a roadside speed display near a pedestrian crossing. 2. Compare that number with your dashboard reading at the same moment. 3. Repeat at two or three speeds, such as 30, 50, and 100 km/h, on a straight road. Users in the thread also suggested OBD speed as a second comparison. This method avoids relying on one phone app alone. [#21348775]

4. Which is more reliable for measuring real vehicle speed: the car speedometer, GPS navigation on a phone, an OBD scanner, or a roadside speed display?

For real speed, a steady GPS or a calibrated roadside display was treated as closer than the dashboard speedometer. The car speedometer is intentionally biased high, while OBD speed can still differ by make and software. One user said their calibrated electronic meter, GPS, and roadside display agreed, with the display reading within ±1 km/h at 30 km/h. The most reliable result comes from comparing at least two methods on a straight, constant-speed run. [#21349005]

5. How do tyre wear and tyre diameter changes affect speedometer readings over time?

Tyre wear makes the speedometer read higher over time. As tread wears down, tyre diameter and circumference both shrink. That means the wheel rotates more times per kilometre, so the system interprets the car as moving faster than it really is. The thread states this factory error margin exists partly because tyre wear changes circumference during normal use. [#21348771]

6. What happens to speedometer accuracy after changing to larger wheels, different rims, or low-profile tyres?

Changing wheel and tyre size can reduce the built-in over-read or even make the speedometer read too low. The thread explains that larger rolling circumference means higher real speed for the same wheel rotation count. Users specifically warned that bigger wheels with low-profile tyres can make the dashboard show less than actual speed, which can push the car outside the intended approval range. [#21348976]

7. How accurate are roadside radar speed displays near pedestrian crossings compared with GPS and the car's dashboard speedometer?

They were described as quite accurate, but not perfect. One user said those roadside displays often show nearly the same speed as GPS, and another reported about ±1 km/h at 30 km/h. Other posters cautioned that every measuring device has error, including police radar and roadside displays. So they are useful reference tools, not absolute truth. [#21349005]

8. What is OBD vehicle speed, and how can I use an OBD scanner to compare ECU speed with the dashboard reading?

“OBD vehicle speed” is a diagnostic data value that the engine or body controller reports as vehicle speed, usually through a scanner connected to the car’s onboard diagnostics port. It lets you compare ECU-reported speed with the dashboard display. In the thread, a user advised plugging in any OBD scanner, opening the vehicle-speed parameter, and checking it against the instrument cluster at a steady speed. They also noted ECU speed often reads lower than the dashboard, and the amount varies by make. [#21349059]

9. What is UN ECE Regulation R39, and how does it relate to car and moped speedometer over-reading rules?

In the thread, R39 is the cited international regulation used to justify speedometer over-reading limits for vehicles. Posters linked it to the rule that the display must not under-read and may over-read within the quoted formula. One user added that mopeds should also overestimate by up to 10% plus 4 km/h, and that moped speedometer scales should not exceed 80 km/h. [#21368859]

10. Why can two different smartphones show different GPS speeds in the same car on the same road?

Because phone GPS speed is not just about satellites; the device matters too. One user who tested several phones saw differences from 3 km/h to well over 10 km/h in the same car. They blamed GPS fix quality, with weaker phones showing worse results. Another poster said some phones and moments match the car exactly, while others drift above or below it. [#21351385]

11. How do road shape, satellite visibility, weather, and phone GPS chipset quality influence GPS speed accuracy?

They can change GPS speed noticeably, especially on winding roads or in poor reception. The thread names straight versus winding roads, tall trees, rocks, buildings, weather, satellite count, and processor class as factors. Those conditions affect the quality of the GPS fix and can make the displayed speed lag or vary. That is why the same phone can agree with the car at one moment and disagree a minute later. [#21348931]

12. At 160 km/h on the dashboard and 148 km/h on GPS, how should I interpret the difference and estimate the real speedometer error?

Interpret it as an indicated over-read of about 12 km/h, assuming the GPS had a stable fix. The thread gives exactly that example: 160 km/h on the dashboard and 148 km/h on GPS. That is a gap of roughly 7.5% relative to 160 km/h indicated. It does not prove the GPS is perfect, but it strongly suggests the car was travelling closer to 148 than 160 km/h at that moment. [#21348789]

13. Which Polish or international regulation governs speedometer indication accuracy and testing for passenger cars in Poland?

The thread points to the international UN ECE framework, specifically R39, rather than a separate Poland-only rule. Users also cited an EU approval document and stated the detailed speedometer error rule appears in section 5.3 of the attached PDF. In practice, the thread treats that international approval rule as the governing reference for passenger-car speedometer indication accuracy. [#21348931]

14. How is a digital or stepper-motor instrument cluster calibrated, and what diagnostic mode checks can verify 50 km/h and 100 km/h indications?

A digital cluster with stepper motors can be checked through its built-in diagnostic sweep mode. One poster noted that “smart” clusters often do not rest perfectly at zero when powered on, which already shows electronic control and calibration behavior. They added that diagnostic mode can command test positions such as 50 km/h and 100 km/h, letting you verify whether the pointer lands correctly at those marks. [#21350381]

15. What is ERTO tyre sizing, and why might the manufacturer's quoted tyre dimensions differ from the real rolling circumference used for speedometer calibration?

“ERTO tyre sizing” is a tyre-dimension standard that specifies nominal tyre size, but it does not directly equal the real rolling circumference under load, pressure, and motion. One user said the manufacturer’s quoted ERTO value did not match the true rolling circumference, so they calibrated their digital speedometer to the tyre’s actual circumference while rolling. That is why a speedometer can be accurate only after real-world calibration, not just catalogue sizing. [#21349005]
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