Electronic speedometers are fairly accurate - when everything that the factory calibrated them for is still the same. When you start changing things in your drive train, what it says on your dash and what it says on the nice policeman's radar gun can be quite different.
We now offer a small, easy to use module that wires in between your speed sensor and the speedometer, and allows correction to put the readout back where it should be. Using a single multi-turn pot, stepless adjustment can be made from 2.5% to 250% of the input frequency. Calibration test points inside the housing allow the use of a multimeter to quickly dial-in the adjustment.
The Speedo Corrector sells for $60.