Speed Limit Setting Mystery?
 

Hello all.

First time I've played with the Speed limit thingy in the MMI and have an odd thing going on.

I set it at 60mph and tried it out. I didn't get the ding or warning at 60mph or just over.

The speedo kept going to approx 67 - 68mph, and then I got the ding and red warning symbol at the top of the DIS.

So I set it to 70mph and had another run. The ding and warning went off at approx 77 - 78mph.

The 8 came with 20 inch alloys and 265x40x20 tyres. Because of this, I think the speedo was out slightly, eg: Speedo reading 70 mph, when the car was only doing around 65 mph, I think.

It's now on the original spec 17 inch alloys and 235x55x17 tyres.

Do you think all this, original spec 17's to 20 inch wheels and back to original spec 17's has messed the speedo up and giving false readings when the speed limit is set?

Speedo or MMI speed limit setting, which is the true reading I wonder.

Hope all that makes sense, ta.

Being on the incorrect wheels would have definitely given an incorrect reading, unless the car was 'told' is was on 20" at the time. If it was coded then the car now thinks you are on 20" wheels and hence the speedo is out.

A quick search, I found this for VCDS

or reference, the VCDS coding slot:

17 - Instrument
byte 3
bits 0-2
"tire circumference"

If you don't have VCDS, I would invest in one as they are invaluable for our cars or indeed any VAG car

Yes but if the speedo is out, so would the speed limiter be. :Confused:
The problem implies the speed limiter is either using a different data source to the Speedo display or, more likely, doesn't cut in absolutely.

Other D3 drivers who use this feature may be able to advise, if there are any here......

The electronics know exactly what speed the car is going, but the speedo is usually deliberately over actual speed by a few %. The trip computer tends to be the most accurate bit of the user interface - just measure average speed for a few seconds.

So the message is, set the tyre circumference correctly if you want to rely on the speed limiter.
(And assume the speedo display is probably a bit optimistic.)

I think on a D2 (and I know this is a D3) the only place to set that is in the RNS. But D2's don't have speed limiters.

Speed warning is in the DIS +++

True, and it's accurate, unlike the speedo. But it only beeps and lets you carry on. :D

My D3 speed warning is set at 100mph and beeps at exactly 100mph on the speedo (when tested on a private track obviously).
As said previously I would start at wheel circumference calibration.... see what it’s calibrated as and check what your wheels are etc

Ok, thanks for your input everyone +++

I'll see what VCDS says and report back.

Just to clarify, does the D3 have a limiter or just a warning like the D2, or both?

Warning the same as the D2 +++

Hence I misunderstood the initial question, which I read as if the car was supposed to stop accelerating at the set limit. :o

Yes, just a ding and red warning symbol at the top of the DIS. It disappears when the speedo goes above the set speed limit warning.

In my case:

Set at 60mph.........warning at 67mph, approx.

Set at 70mph.........warning at 77mph, approx.

Speedometer v speed limit...............one of them is fibbing........maybe both!

At least the warning mph's are consistent :D

Ok, had a look at VCDS - 17 - Instruments.

http://forum.a8parts.co.uk/attachmen...1&d=1616766196

Went to Coding - 07. This popped up over the coding:

http://forum.a8parts.co.uk/attachmen...1&d=1616766274

Clicked that off and here's the coding:

http://forum.a8parts.co.uk/attachmen...1&d=1616766372

Couldn't figure out how to get to the 'bits' and 'bytes' part.

Had a look at this, starting at 1.28:

https://www.youtube.com/watch?v=W4PHXY-K9G0&t=110s

Seems my VCDS doesn't have any 'long coding helper' for the wheels :Confused:

The only place I know that you can input any wheel data is in Adaptation Channel 01, Nav module. Format is wheel circumference in MM. It's supposed to be used to correct the navigation. It may or may not be used by 0x17 for the speed limit gong. Worth a try changing the value.

I'm bound to mess something in there Matty :o

Could you explain that in more detail please? In dummy language!

Anyway, how come the bloke in the vid has the wheel stuff on his VCDS and I haven't? I thought all VCDS would be the same?

This is my version:

VCDS Version: 21.3.0.0 (x64) HEX-V2 CB: 0.4543.4
Data version: 20210226 DS325.0
www.Ross-Tech.com

You'll be fine. Going from your previous posts you're doing well with VCDS.

Open 37-Navigation, then Adaptation, read channel 01, write down the current value just in case you may need it, then enter a new value (this will be you wheel circumference in mm), click Test then Save.

The guy in the video can access that info only because the control module lets him. He was working on a newer car platform and it is dependent what is written in the firmware, so it's not a feature missing from your VCDS. The software / firmware in the D3 is generations old now.

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.

Well the Nav unit does want to know the wheel circumference and number of ABS wheel impulse. But like I said previous, it is for correcting the navigation. Whether or not this value is broadcast so other modules use it, I don't know. But it's worth changing the value if it makes any difference.

Yes. Although you could experiment with obviously small and larger values.

Ok, thanks Matty, I'll give it a go and report back.

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.

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.

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:

http://forum.a8parts.co.uk/attachmen...1&d=1616865572

Value that was already stored:

http://forum.a8parts.co.uk/attachmen...1&d=1617376582

New value:

http://forum.a8parts.co.uk/attachmen...1&d=1617376664

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:

http://forum.a8parts.co.uk/attachmen...1&d=1616867377

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 :ROFL:

Can anyone decipher all that please?

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 :o:D