I find that there is a designed error in most older car's speedos. I can tell you the exact error in my bimmer. It is 1.5 mph plus 2.5% of your speed. My friends e34 535 has the same exact error as well. The C class seems to be dead on with the accuracy.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record