Speed Limit Setting Mystery?

[HPsauce]

The wheel rotations are used for "dead reckoning" by the SatNav in between GPS fixes, e.g. in tunnels, car parks etc.
Not sure how (if at all) it's used by the speedo/instruments though.
I was told many moons ago that the yaw and other direction sensors are also used on a D2 (RNS-C and RNS-D), but I'm not sure that's true...
I've never bothered changing it (though I have checked it's sensible) for different wheels/tyres, e.g. summer/winter as the diameter (and hence circumference) is pretty similar.

[Echo]

Originally Posted by HPsauce

The wheel rotations are used for "dead reckoning" by the SatNav in between GPS fixes, e.g. in tunnels, car parks etc.
Not sure how (if at all) it's used by the speedo/instruments though.
I was told many moons ago that the yaw and other direction sensors are also used on a D2 (RNS-C and RNS-D), but I'm not sure that's true...
I've never bothered changing it (though I have checked it's sensible) for different wheels/tyres, e.g. summer/winter as the diameter (and hence circumference) is pretty similar.

Thanks for the info HP.

Seeing as there's nowhere else in VCDS (on the D3), to sort out wheel settings, I'll give Matty's Nav module adaption a go.

I'll be back.

[ainarssems]

Originally Posted by Echo

Ok, thanks Matty, I think I've got that.

But what will changing the wheel circumference in the Nav module actually do?

I've put my new(17"), and old (20"), tyre details into this calculator:

https://tiresize.com/comparison/

That was inches. This one is mm:

https://tiresize.com/comparison/

So, I put in 2167.........yes/no?

And thanks for explaining the Youtube VCDS stuff

EDIT: Sorry, can't get the links to work properly. Just click on the inches and mm to compare.

First of all I would be very surprised if Satnav sends back speed reading to instrument cluster, any adaptation you do in Satnav will only be for Satnav.

The most accurate way would be to get a piece of string and actually measure circumference of tyre as it could be slightly different between between different makes and models. Wear level would affect it as well and different tyres have different tread depth when new. Most are 8mm but some winter tyres can be up to 12mm and Michelin Crossclimate range only have 7mm when new for some reason. But they still offer 45k mileage warranty for CC, 50k for CC+ and 60k for CC2. How they make them last so long with less tread to start with is dark magic and how you go about claiming it is a different question.

Now back to speed issues I am wondering if there is adaption channel in instrument to alter speed reading, I know you can adjust things like fuel level reading or fuel consumption by +/-15%. As far as I know it's not possible for D2 but some VWs have a multiplier option there, not sure about D3.

I never use speed warnings but I remembered about this thread when I went out this morning and tested mine. I have 2006 car with wide instrument cluster so I have digital speedo reed out as well which makes it easier. The tyres on car are almost new 235/50R18 Crosslimate +, done about 3k miles. I do not know what the stock size was on this car but I can see that wheels are not original but from newer Audi replaced by one of previous owners as they have bigger centre bore 66.1mm instead of 57.1mm and use spigot rings. Digital speedo and analog show the same speed, comparing it to GPS reading in the phone speedo is showing 1-2mph more than GPS at 70mph.

Setting speed limit with 'Çheck' button at 70mph warning comes on at 72mph but there is a bit of error do to manual setting. Setting speed limit to 70mpg in MMI the warning comes on at 71mph

[Echo]

Originally Posted by ainarssems

First of all I would be very surprised if Satnav sends back speed reading to instrument cluster, any adaptation you do in Satnav will only be for Satnav.

The most accurate way would be to get a piece of string and actually measure circumference of tyre as it could be slightly different between between different makes and models. Wear level would affect it as well and different tyres have different tread depth when new. Most are 8mm but some winter tyres can be up to 12mm and Michelin Crossclimate range only have 7mm when new for some reason. But they still offer 45k mileage warranty for CC, 50k for CC+ and 60k for CC2. How they make them last so long with less tread to start with is dark magic and how you go about claiming it is a different question.

Now back to speed issues I am wondering if there is adaption channel in instrument to alter speed reading, I know you can adjust things like fuel level reading or fuel consumption by +/-15%. As far as I know it's not possible for D2 but some VWs have a multiplier option there, not sure about D3.

I never use speed warnings but I remembered about this thread when I went out this morning and tested mine. I have 2006 car with wide instrument cluster so I have digital speedo reed out as well which makes it easier. The tyres on car are almost new 235/50R18 Crosslimate +, done about 3k miles. I do not know what the stock size was on this car but I can see that wheels are not original but from newer Audi replaced by one of previous owners as they have bigger centre bore 66.1mm instead of 57.1mm and use spigot rings. Digital speedo and analog show the same speed, comparing it to GPS reading in the phone speedo is showing 1-2mph more than GPS at 70mph.

Setting speed limit with 'Çheck' button at 70mph warning comes on at 72mph but there is a bit of error do to manual setting. Setting speed limit to 70mpg in MMI the warning comes on at 71mph

Yes, thanks Ainarssems.

Really, all this speed warning stuff had got me wondering if my speedo needle was reading accurately with changing the wheels from 20" to 17".

I don't know if there is an adaption in the instruments to change anything either, but, as previously stated further back in this thread, there aren't any
'bits' and 'bytes' in my VCDS to play with to change the wheel size.

Full report in my next post.

[Echo]

Reporting back

First of all, as Ainarssems suggestion, I measured the tyre with string.

The online tyre (although it says 'Tire'), comparison calculator indicates that, for 235x55x17 it should be 2167mm circumference.

https://tiresize.com/comparison/

With the string, it was approx 2130mm. Seeing as I couldn't see underneath properly, and hoping the string wasn't in any of the tyre grooves, I added another 10mm for good luck! So, 2140mm circumference.

I then gave Matty's Nav module adaption a whirl:



Value that was already stored:



New value:



What do the different values of 2105 V 2140 mean concerning the wheel size?

Went for a drive around.

The speed warning (set at 60mph), came on when the speedo needle hit approx 67mph, as it usually does

Although, not positive, but I think the nice lady gave me a bit more warning of upcoming slip roads and junctions. So the change in values might have done that?

Whilst driving, I had a go at trying to compare speed from different sources.

I don't think I have the digital speedo readout, so I used the km/h vehicle speed in VCDS - Instruments as I was driving:



When the speedo needle was (approx), dead on a certain speed, I had a VERY quick glance at the VCDS. This is what I observed:

1. Needle at 20mph - VCDS showing 30kmh.

2. Needle at 50mph - VCDS showing 77kmh.

3. Needle at 60mph - VCDS showing 92kmh.

4. Needle at 70mph - VCDS showing 109kmh.

According to the Google mph/kmh calculator, the above should look like this:

1. Needle at 20mph - VCDS should show 32.1kmh.

2. Needle at 50mph - VCDS should show 80.4kmh.

3. Needle at 60mph - VCDS should show 96.5kmh.

4. Needle at 70mph - VCDS should show 112.6kmh.

Or, to put it the other way round, the Google calculator says it should look like this, from the first 1 - 4:

1. VCDS showing 30kmh - Needle should read 18.6mph.

2. VCDS showing 77kmh - Needle should read 47.8mph.

3. VCDS showing 92kmh - needle should read 57.1mph.

4. VCDS showing 109kmh - needle should read 67.7mph.

So, 60mph speed warning comes on at approx 67mph.
67mph = 107.8kmh.
107.8kmh = 66.9mph.

I've gone all wobbly and confused and my head hurts now

Can anyone decipher all that please?

[Echo]

Did a few more tests.

Started a stopwatch, and at the same time, zeroed the trip meter above the mileage on the dash.

Results covering 1 mile, according to this calculator:

https://www.gigacalculator.com/calcu...calculator.php

Needle on 40mph - 1min 35 secs - calculator says 37.8mph.

Needle on 50mph - 1min 15 secs - calculator says 48mph.

Needle on 60mph - 1min 5secs - calculator says 55.3mph.

Other online calculators say different speeds. This one:

https://www.calculatorsoup.com/calcu...calculator.php

...........is 4mph over the 40, but says 57mph on the 60. You'd think calculators would all be the same eh!

Anyway, did another test.

This time with an assistant, with Google Maps GPS on their phone.

Did 3 runs at each speed over 1 mile:

Needle on 40mph - Google Maps GPS - 38mph 37mph 37mph.

Needle on 50mph - Google Maps GPS - 48mph 47mph 47mph.

Needle on 60mph - Google Maps GPS - 57mph 56mph 56mph.

Pretty consistent there then I think, and looks like my speedo needle is reading 2 - 3mph over, which is what it's supposed to do........right?

It still doesn't solve the mystery of the speed limit warning, set at 60mph, but going off at 67mph. That would mean the car is doing around 64mph.

I then set the speed limit warning at 55mph........it went off at 62mph.

Still as confused as ever