Speed Limit Setting Mystery?

[Echo]

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.

[homer simpson]

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

[HPsauce]

Yes but if the speedo is out, so would the speed limiter be.
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......

[MikkiJayne]

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.

[HPsauce]

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.

[MikkiJayne]

Originally Posted by HPsauce

But D2's don't have speed limiters.

Speed warning is in the DIS

[HPsauce]

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

[89forever]

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

[Echo]

Ok, thanks for your input everyone

I'll see what VCDS says and report back.

[HPsauce]

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