Hi guys!I'm new here,glad to find you!I suppose many people have already thought of it but a smart way of measuring your real speed and the speedo's mistake is this:
Find a quite flat empty road with no "serious" corners and use the cruise control to maintain a stable speed. eg 100kmh(speedo).Then reset the trip computer TRIP B function and let it calculate the real speed.As it measures distance and time (with a lot better precision than the speedo measures speed) and divides them,it calculates your average speed pretty precisely.after a few kms (eg 3-4kms)the precision will be very good.I tried it out and here are the results:
SPEEDO: 100 REAL SPEED:96
SPEEDO: 105 REAL SPEED:100
SPEEDO: 147 REAL SPEED:140
Hope to have helped.And please excuse me English,it's not really good...