[blykins]

You use a driven speedometer gear. If you follow the end of your speedometer cable to the transmission, it attaches by one bolt. If you pull that out, there's a little gear on the end of it. You swap that gear to get your speedometer to read right.

Post the rearend ratio of your car, along with the rear tire height, and I'll tell you which gear you need. They're cheap and quick to change.