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?