GPS speedo works great when you have a clear view to the south to catch the satellite signals. Doesn't work very well at all when you don't have that view, such as in a tunnel, forested area, mountainous region and so on. This is second hand info from a guy I know.
As for the mechanical speedo, if you or someone you know is handy with electronics, you might be able to rig up a stepper motor driven by a hall effect sensor on the wheel. The rotation of the wheel would produce pulses that would be read by a computer and output as instructions to the stepper to spin. The stepper could be connected to the mechanical input on the speedo. Actually, just replace the speedo and chock up the loss to experience gained.
