Not sure about BMW but VW/Audi/Seat/Skoda can have the accuracy improved by a correction factpor being applied by the dealer. I had a speedo replaced a few years back and the unit was over-reading by about 15% so the dealer 'corrected' it to within 2mph at 70mph. The change was made by changing a value in the ECU/Vehicle instruments computer or whatever system actually drives the speedo itself (not the wheel speed sensor which is just the input device).

In the UK speedos are allowed to over-read but never under-read with the limit being set at 10% +/-2mph @ 30mph on a calibrated test system.

My speedo indication is about +3mph @ 70mph when compared to a GPS system.

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Me:
Current: 2011 E71 40D Vermilion Red/Beige Nappa

Wifey:
Current: 2012 R80 Countryman SDX Green/Cream