How about reading 0 when you're actually driving 5 MPH?

 

OK, one experiment is worth dozens of speculations.
I took my 2011 Clubman out for a test drive, reading actual speed from a Garmin GPS, and indicated speed from the digital display in the tachometer. The results are as follows:

Garmin MINI
0 0
10 11 mph
20 22
30 33
40 43
50 53
60 63
70 73
80 83


There was too much traffic to try higher speeds. So _my_ MINI has a constant 3 mph excess at usual speeds. As they say, YMMV.

 

Our 09 MCS is the same...3mph slower than actual.