Speedometer Accuracy

[Exkiwi]

2 hours ago, Avocet said:

It is possible to alter speedo in some VAG cars   I altered mine in a 2019 TIguan but since then VAG has crippled a lot of things you can do with VCDS or OBD Eleven.

 

The speedo works by impulses from the ABS sensor in a rear hub. It is set for the recommended tyres to give a certain number of impulses per Km (or Mile ) depending on the area its sold in.

 

 

In the TIguan the OBD Eleven had a place where it had 6 alternatives for wheel size    I altered it from 3 to 1 and it brought speedo almost bang on a t 100kph   I havent found the same available for the Kamiq   Just stick to the perceived speed and you wont get a ticket.

 

 

 

 

 

 

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[J.R.]

GPS speed indications are only valid for a level road are they not?

 

On an incline or descent they would in my mind under-read.

[linni]

5 hours ago, Rooted said:

 did not say if the replacement Octavia driven last year was on the OEM tyres as it left the factory.

 

 

Definitely on original tyres. Dealer`s car, cannot see any reason why they had swapped the wheels.

[linni]

2 hours ago, J.R. said:

GPS speed indications are only valid for a level road are they not?

 

 

Estonia is nothing more than a plainland.

2 hours ago, J.R. said:

 

On an incline or descent they would in my mind under-read.

 

No. GPS takes in consideration also the distance to satellite. And even if it didn`t, the error margin is basically zero, driving the car. 

[Stonekeeper]

2 hours ago, J.R. said:

GPS speed indications are only valid for a level road are they not?

 

On an incline or descent they would in my mind under-read.

 

I agree with your logic.

 

Also even bends will make them slightly inaccurate according to some reports. ( i might check this driving round a roundabout 😂 )

 

if the system only places you to a point within 10metres, as you move you would be sort of jumping about a bit and it needs more than one satellite to find your position. So cannot be 100%.

 

 

I rely on my speedometer and try to not go more than 10% above the speed limit.

 

 

 

 

[J.R.]

1 hour ago, linni said:

 

the error margin is basically zero, driving the car. 

 

Yeah right!!!!!!!!!!!!!!!!!!!!!!!

[linni]

Just now, J.R. said:

 

Yeah right!!!!!!!!!!!!!!!!!!!!!!!

 

Seems you have issues with elementary mathematics.

Margin can depend only on your device`s accuracy not driving uphill or downhill.

[Routemaster1461]

Having set up a digital speedo in a vehicle, I know that the reading can jump all over the place, depending on how many satellites can be seen. And a satellite speedo would be absolutely no good in tunnels where there is no satellite coverage.

[Winston_Woof]

NB GPS based speedos are not a legal replacement for conventional speedos

[Ootohere]

There are plenty towns and villages in Scotland or Straights with Digital Speed Signs with a frown or a smilly face and a speed read out.

An accurate speed if you check with what ever.  GPS / Sat nav, GPS / app on a watch or phone, a V-Box etc.  

[Avocet]

14 hours ago, J.R. said:

GPS speed indications are only valid for a level road are they not?

 

On an incline or descent they would in my mind under-read.

As others have said, I think the error would be low, unless falling off  a cliff.  They prefer straight (ish) line speeds, though.  If you dove in a tight enough circle, the GPS speed might show zero, but I don't think it's possible to find a car with a tight enough turning circle.  By and large they're good enough on a reasonably straight road or even one with shallow bends.  Certainly more accurate than the speedo.

[J.R.]

18 hours ago, linni said:

 

Seems you have issues with elementary mathematics.

Margin can depend only on your device`s accuracy not driving uphill or downhill.

 

Someone does Linni and it isn't me!

[Stonekeeper]

From a GPS forum,

 

 

[J.R.]

I knew the mathematics, I knew the Cosine Theta error would be small on most road gradients, I also knew that anyone that speaks of anything being "spot on" or "margin of basically zero" would neither know nor understand so I was happy to leave it unexplained and them blissful.

 

It was amusing to be told that I have issues with elementary maths though 😁

[Yogi-Bear]

I may be talking rubbish here, as I’m no expert on GPS… but my understanding is that 3 satellites are required for a 2D fix (long + lat), and 4 or more for a 3D fix (+ altitude). So if the GPS has a 3D fix, it can measure changes in all 3 dimensions, and can therefore show true speed in whatever direction you’re going - even if it was vertically off a cliff.

[linni]

5 hours ago, J.R. said:

It was amusing to be told that I have issues with elementary maths though 😁

 

 

Yes, looks like it. Some can explain things, some can only mouth.

 

10% slope angle corresponds appr to 5,7 degrees. So, 10 degrees correspond to 18% slope angle. I have been travelling all over Europe, never seen this kind of a gradient. I have a 7% downhill near my country house and this seems like diving straight down.

If I remember correctly, the max slope can be 15% in Europe and modern roads are built using max 5% slope. Steepest road in Europe is in Norway and has 20% slope.

 

What we are talking here is that in real life the slope error can usually be 0,1 - 0,2% of the speed.

 

So, driving at 90 kmh the max slope error will be 0,18 kmh. 

 

Conclusion - can we trust GPS readings on down- or uphills? Yes, we can.

 

 

 

[Yogi-Bear]

32 minutes ago, linni said:

If I remember correctly, the max slope can be 15% in Europe and modern roads are built using max 5% slope. Steepest road in Europe is in Norway and has 20% slope.

20% is for wimps. Try North Wales (Harlech)…