I don't know about the TD5 speedo, but I do know the BMW electronic speedo on my ex-police K1100lt (bike) can be adjusted and, as it works upon the same principal as the TD5's electronic one I would assume the same is possible. Somewhere I had a bookmark for how to set it up but can't find it now. I know it hasn't been checked for a long time, but the speedo head on my bike has a calibration stamp on it from when it was a police machine so I usually use that as my reference (friend on bike at set speed and me driving behind him etc.)
I have found that GPS are not very accurate. I assume it is something to do with the fact that they are accurate to within so many meters of your location, the time taken to make the necessary calculations and the time taken to display everything on the screen. I find my GPS shows about 2 to 3 MPH less than my speedo, and the roadside speed displays in small villages etc.
I agree with those who say that your vehicle speedo is probably set anything up to 10% high. Otherwise, every time you got done for speeding, you could sue the manufacturers if it was showing under.
Can't you just tie a knot in the wire to slow down the pulses?