I'm on stock rubber in good condition and GPS is bang on the speed sensor
output, albeit I have to express GPS in kph to get the same number.

If VAG are consistently underestimating speed to the EMS this must have some
fairly important ramifications to the EMS maintaining engine tune, etc? Unless
it's all relative - it could be done that way.

The introduction of error for the speedo is fair enough, generally the
adjustment means that in any extreme circumstances, you will still read no
lower than the true speed and can't blame the car under reading when the
rozzers catch you.

There doesn't *seem* to be any similar reason to under read. I suspect an
undesigned fudge has crept in, given that other EMS doesn't also do this and
any design considerations can be introduced deliberately at the EMS end.

I'm thinking it might be something of the sort where the sensor was designed to
read revolutions of the gearbox output shaft, and the EMS was designed to read
these pulses from the sensor based on, say, 13 inch wheels with 195/65 rubber
fitted. And, in those circumstances, the sensor pulses translate into, say, 50
mph inside the EMS.

Then someone took all these parts from the parts bin, and fitted them to a car
that marketing has decreed shall run on 14 inch wheels with 215/45 rubber or
something of that sort, where it means that the standard EMS and sensor now
read 48 where speed is actually 50. Since there is still +5 mph added on for da
Guvvamint the speedo now reads 53 so it complies with legislation just fine.

I have a feeling this would hit the fuel consumption measurements, but probably
this makes no difference apart from slightly misleading the driver.

I'm probably over-analysing this. ;)