There is a calibration factor on the back of the speedometer plate (its a three digit number as I recall) - another speedometer can be recalibrated on the test bench at a speedo shop where they know what they are doing, using that factor to set up another replacement speedo that you then install yourself.
Or they can recalibrate your new speedo on the car using a Dyno (more accurate) - costs more this way of course but it should be more accurate. If you don't have a speedo shop locally, there are several that you can send the units to for calibration service, there was one in Austin TX, maybe R Leo knows them. On the west coast we have Palo Alto Speedometer as the chief place for VDO service. If I knew what little thing to tweak I would try it myself, hit or miss kind of thing, but since I never have much patience when working on instruments I leave that to the experts.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!