[sblument]

I got curious and just went out and did a test. I have a 2006 330i sport pkg with winter tires on (Dunlop Wintersport M3 DSST, 225/45HR17).

I set the cruise control on 25, 30, 35, 40, 45mph and waited for speed to stabilize on reasonably smooth, straight, level road. I went to the BC and looked at average speed, resetting it once I was up to speed so I could see the instantaneous speed. The digital speed and the analog speed on the speedometer were never more than .3mph apart, and were often .1 or .2mph apart, with the digital being lower than the analog.

I feel that the analog speedo is off by about 5mph (reading too fast) with the winter tires, and 2-3mph too fast with the summers, at speeds in the neighborhood of 45mph