I believe one of the septics on the ADV site has investigated the inaccurate speedo and managed to cut the inaccuracy from about 7% to about 2% using a speed drive from an R or RS model.
Seeing as how the drive for the odometer is the same as the speedo,i would say that there is a stong possibiliy that the odometer is out by the same margin.
A GPS unit that shows speed and distance travelled could confirm your suspicions.
Col