here are some additional numbers from the car's internals -

4282 wheel sensor impulses to register 1 kilometer in odometer
4229 wheel sensor impulses to register 1 kilometer in speedometer

these are the vallues stored in the E53-HP, which is the iS model with 20" standard staggered wheels... these numbers suggest that the rolling values of the mounted tires are different from the nominal values that I got from the tire calculators - 233.53cm as per the 4282 impulses in odometer ("correct" value as opposed to optimistic 4229 impulses for the speedometer value)

these numbers do not affect the calculations of the MPG much, as we used our numbers to calculate the difference in diameter and those numbers were a percentage points of a ratio...

nevertheless, now i want to go actually measure the tire diameters on my car...