Not sure about Mercedes, but GM speedometer heads are all calibrated to read so many MPH per 1000 RPM of cable speed (can't remember the exact number). Axle and tire variables are handled by the speedo gearing in the transmission and can be changed by swapping the externally removable driven gear.

Mercedes probably uses a similar algorithm, and your best bet would be to contact a local speedometer shop or contact your local dealer and ask them for a referal to the speedo shop they use. A speedo shop that does work for a Mercedes dealer should know the answer, or be able to tell you what needs to be done to make the new speedometer accurate for your application.

Duke