It may be totally unrelated, but I had a similar problem on my Chevy Silverado. Both the speedometer and the tach would bounce and surge, and would usually be about twice as high as they should be. The problem was in a loose ground connection at the battery. I have no idea how the loose ground made the speedo behave that way, but it has happened on two occasions, and on both occasions a good battery terminal cleaning solved the problem. All I can figure is that I was getting a voltage surge somehow.

95 Contour Zetec, ATX (wrecked, sitting in the barn) 95 Mystique Zetec, MTX 2000 Taurus SEL 3.0L Duratec 1994 Crown Victoria LX 4.6 V8 1993 Chevy Silverado 4.3 V6