So the snow tires on the front of my 300m are these:
Cooper Discoverer M+S 215/70 R 16
They're not new (so maybe missing 1/16" or 3/32" worth of tread).
Only about 1/4" clearance between the tire and shock-tower guard.
Recent highway trip (about 100 miles) and 6 data points correlating highway mile-markers and odometer readings show that for every mile (or km) being travelled, that the odometer is turning over 0.9655 miles (or km). Correlation coef. for the data is .9999. Road conditions were dry, no snow. Speed was between 65 and 70 mph constant as set by cruise control (110 km/hr).
The circumference of these tires are therefore about 3.57% larger than the stock or reference tire, which is 225/55 R 17.
Speedometer correlation with GPS indicates that the speedo gives a perfectly correct speed reading with these tires (not an under-estimation as you would expect).