Difference between my SatNav and the Z3 was about 8%. I fitted slightly taller tyres and now it's 3%
On my Merc the OEM installed SatNav is always exactly the same as the speed on the dash
I had a Mazda6 a few years ago that had HUD which showed the speed. It also had an analogue (normal speedo). The two always read different speeds which was very annoying, especially as the HUD was directly above the analogue speedo.
The other thing that car had a habit of doing, was changing the satnav language, on its own, into one which had a different alphabet to ours. Never sorted it despite several 'updates' done by the stealer.