Speedometer problem - 3.6 litre XJSC

First of all, thanks to all for the useful advice concerning the electrical issue with the dash instruments. When I remove the cluster to install a new ground, I would also like to solve the problem with the speedometer. The car was sold new in Germany, imported to the States and federalized. The speedometer was replaced with an aftermarket speedometer that read only in MPH. The only complaint I received when I bought the car from the original owner was that the speedometer was inaccurate. When the speedometer read 75 MPH, the car was actually going more like 50 MPH. The odometer was equally inaccurate which was no surprise. I found a replacement speedometer from a 3.6 XJS import which reads in KPH, but obviously has the MPH numbers on the dial as well. It seemed appropriate to me, considering that the car came from Germany.I had the mileage reset by a speedometer shop to reflect the actual mileage on the car as well as checking it out. It came back with a clean bill of health, but has two issues. It doesn't start to work until the car is going about 15 MPH, and it is also inaccurate in the opposite direction. When the car is going 75 MPH [about 120 KPH], the speedometer reads about 50 MPH. Is there a sending unit or some other part besides the speedometer itself, that I need to deal with? Should I have the speedometer recalibrated somehow? Is replacing the current speedometer the way to go? Your expertise and advice will be greatly appreciated, once again. Thanks - Todd Stuart