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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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  • #31 21350381
    BANANvanDYK
    Level 42  
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    Has no one noticed that in "smart" instrument clusters, i.e. microcontroller-controlled and with stepper motor-driven clocks, the hands do not rise perfectly to the "0" dash when switched on? Besides, meters in diagnostic mode have the possibility to check the correct functioning of the indicators, e.g. indication of 50 km/h, 100 km/h.
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  • #32 21350394
    Mastertech
    Level 30  
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    This is why it is so difficult to put a pointer on a clock with a stepper motor,
    As the author's colleague told me, it is not the same for him as for everyone else.
    In newer cars which have a factory GPS, neither the speedometer nor the navi shows the actual value from the GPS, but only after conversion according to the formula which has already appeared here in the topic.
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    #33 21350640
    Anonymous
    Level 1  
  • #34 21351385
    Ordine
    Level 25  
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    For me, depending on the phone - of which I have several - it varies between 3km/h from the speedometer to well over 10. It all depends on how realistic the GPS fix is for the phone. Chinese inventions have the biggest problems with this.
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  • #35 21351395
    kulmar
    Level 33  
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    0 ≤ V1 - V2 ≤ (V2/10) + 4 km/h
    where V2 is the speed actually reached by the vehicle and V1 the speed reported on the clocks.

    This formula has a simple interpretation: for high speeds, the error in the speedometer display (excess) must not exceed 10% of the actual speed; for low speeds, the error must not exceed 4km/h.
  • #36 21351843
    urkotrebor
    Level 21  
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    When it comes to overestimation of speedometers in new cars there is nothing to discuss because it is a fact .
    My observations indicate that it is from 5 to 10 km / h and I have not encountered that the speed is underestimated (unless the meter is damaged), the subject did not occupy me so much to check exactly, but I have the impression (not supported by measurements) that American vehicles do not have this affliction (maybe someone can confirm or deny it) .
    In my opinion, however, this defect is not a problem because I know about it and because I do not have a temperature gauge in my car I built myself a device that reads the speed from the ABS and some other values .
    The reading from the ABS in my case seems to be accurate and coincides with the speed tables in my area .
    Now to the point , after the introduction of the new fine tariff I observe how the average speed has dropped drastically , because the meter overestimates (5-10) , because I will pay (10 and more) .
    Traffic jams form in my area, journey times have sometimes increased by 100%, CO2 production has increased because it is still 2nd gear in gusts to 3rd gear, leaving the subdivision is a problem.
    The fact that fewer people die on the roads, but what the future holds (lung diseases, heart disease, osteoarticular problems) .
    And although there are fewer victims, there are more bumps and collisions, because at low speed people become less careful .
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  • #37 21356268
    Anonymous
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  • #38 21356298
    James596
    Level 29  
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    But there are not that many "no limit" sections in Germany. On city ring roads you're often doing 80 km/h, and there are the ubiquitous roadworks.
  • #39 21356361
    Anonymous
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  • #40 21356378
    Anonymous
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  • #41 21356456
    user64
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    But what does the lack of speed limits have to do with the amount of fines? I'm having a great time there, it's legal to check how much the factory has given.
  • #42 21357083
    Anonymous
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  • #43 21357628
    Anonymous
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  • #44 21368859
    BANANvanDYK
    Level 42  
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    >>21348931 .
    I have read the R39 provisions in detail.
    Interestingly, mopeds should have counters that overestimate by 10% +4 km/h offset, just like in cars.
    In addition, the meters on mopeds must not have a scale exceeding 80 km/h.
    Meanwhile:
    Close-up of a moped speedometer with a maximum scale of up to 120 km/h. .
    This is a 50cc scooter I used to make. The model locked in the worst possible way, that is, structurally in every component, so difficult to unlock. When I rode it, I was surprised to find that it rides like a bike. It's standard for the meters on scooters to overestimate, but I didn't expect it to overestimate so much.
    By the meter it was pulling 50-55, max 60 km/h. I checked my speed with the GPS and it turned out that it was going the legal 45 km/h, max. 50 km/h.
    Basically, the error of the meter readings exceeds the allowed norm, and the red range on the dial of 100-120 km/h not allowed by the regulations is not even possible with this engine.
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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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