I had a long, protracted fight with my odometer, and can now get the instrument cluster out in less than 5 minutes.
Here's the Clark's procedure for early cars:
http://www.clarks-garage.com/shop-manual/elect-06.htm
Here's a calculator to determine how your tire size affects your speedometer accuracy. You input the OEM tire size, then the new size; it shows you the error in MPH.
http://www.discounttire.com/dtcs/infoTireMath.dos
The car is designed to have wheels/tire combinations that are of a certain specific overall circumference. If you decrease the total circumference, the speedometer reads higher than it is; if you increase it, the speedometer reads low.