[lkchris]

This was explained clearly in a service memo for BMW motorcycles back in the 1980s.

"Speedometer advance" is required because it's illegal to produce a vehicle in Germany that has a speedometer that could read less than actual speed. Considered a safety issue (Germans are meticulous). While different brands and models of tires may be marked the same size, that doesn't mean that they are all actually the same size. Of course there is also tread wear as mentioned.

To combat all possibilities, speedometer advance is required.

The bulletin noted that although the speed indicated may be high, the odometer is accurate. Again, how could it be given the possible variances noted? Basically it is what will be used for warranty mileage calculation ... obviously irrelevant today with our 20 + year old 986s.

PS: I'm pretty certain that fitting tires other than approved sizes is an inspection failure in Germany. or was.