Calibrating a speedo?

The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear used with 3.73 rear axle gears works good for some guys, but self destructs in a few months for others. Check out your local Chrysler/Jeep dealer for the gear or find it online.

See Gear Ratio Calc for more help

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap.

Please excuse the typo (theirs, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.

See Speed Correction Devices (Ratio Adapters) for more info.

How the speedometer works:
The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk.

Unfortunately I cannot find the drawing that shows how it all works.

In you case, the magnets may have been weak in the older speedometer.

The speedo heads can be moved from one cluster to the other. It all depends on how much work you want to do. Switch the speedo head and put the spare on ebay for someone else.