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

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!

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).

gerta21w2 wrote:

I was wondering if we can fully trust our car's speedometer, or if these kinds of differences are normal?

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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.

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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

5 km/h is quite a lot.
On mine it misrepresents by 3 km/h

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.

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.

klm787 wrote:

Most vehicles underestimate actual speed by several

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Overestimates, not underestimates, all speedometers have a measurement error of up to 10%.

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."

At my place it is 6km/h kia, 5km/h Mitsubishi. Digital meter.

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"

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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.

No basis. I'm not claiming anything. The cars show more and everything. Don't make up theories.

Jarzabek666 wrote:

you think GPS has an accuracy of 100%?

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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.

@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.

And I write what I see on the display of the car,phone,navigation and speed display in my locality. Just this.

@Aleksander_01
There is detailed information on page 4:
And the formula looks like this:
0 <= (V 1 -V2) <=0.1 V2 + 4 km=h


https://eur-lex.europa.eu/legal-content/en/TXT/?uri=CELEX%3A42004X0331(01)

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 .

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.

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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.

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.

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.

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:
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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?

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.

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

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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.

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.

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.

niewidoczny13 wrote:

o with me it is already outstandingly different.
Because going 160, the gps shows me 148km/h

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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.

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

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I have not heard a bigger humbug.

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.

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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.