Just to chime in here, I'm in the opposite situation as the OP, as my Virtual Cockpit retrofit cluster reads ~10% faster than my GPS speed. (e.g., when the car reads 110km/h, I'm actually going ~101km/h). Stock tire size of 205-55-16 on my 2015 GSW TDI.
I played around with those settings in Byte3 of the Long Coding of 17-Instruments and found that it changed the "Measuring Value" of "Wheel Circumference".
The stock setting (I think it actually even said "standard") gave me these numbers:
I think the full description had "average" option above being the speedometer, with "maximum" being the odometer.
A couple others wouldn't accept the coding; but "Variant 3" (or possibly 4 - I gotta take notes instead of relying on memory
) gave me larger numbers.
Being tired and bad at math, I tried that option, but it doesn't seem to really impact the indicated speed (and granted, it's only around 1% different, so wouldn't do much for my near-10% differential).