As this question comes up from time to time there is now a FAQ on this topic.
e9coupe.com
Speedometer and Differential Ratios
The e9 coupes came with three differential ratios and could be found both open or unlocked and locked or LSD. The speedometers were matched to the car based on which differential was fitted. Differentials The differential ratio is expressed as a fraction and is stamped on the right lower side...
e9coupe.com
