Not being a factory trained HD mechanic, I'm going to base this answer on the information presented so far in this thread.
I am guessing that in 1994, the speedometer is fed by a cable that is connected to a gear somewhere in the drive mechanism, whether it's to the front wheel (common in motorcycles) or elsewhere.
Newer HD's use a sensor in the transmission that sends electrical pulses to the speedometer head, as well as other modules on the bike including the tilt and turn signal module.
Cable driven speedo's have been around since the dawn of cars and bikes, and the cable spinning in the middle drives two things inside of a speedometer: gears that are directly tied to the odometer, and an air/fan type of clutch mechanism that drives the needle to indicated speed.
This mechanism is much like the unit used in an automatic transmission as well, but essentially try to imagine two fans facing each other. One is turned on (the side driven by the wheel of your bike) and it starts spinning. The air created by the movement blowing on the other fan blades will start them turning in sympathy. Now, if you were to put a spring (like a clock or watch spring) on the blades of the second fan, they could allow the blades to only spin so far until the spring is tightened. The spring is calibrated so that depending upon how fast the first fan is spinning, the second one will go further (greater energy transferred causes more movement). Hence, through this type of calibration, you have a speed indicator.
Now, after that simplistic description you can hopefully understand how the speed indicator part of a unit can fail (the fan transfer mechanism fails) and yet the turning of the cable still direct drives the gears on the odometer part.
If you can get inside the unit easily, you might find that some dirt or something is affecting the fan assembly that drives the needle. But, these are highly calibrated units that can easily be thrown way off kilter unless you know what you are doing. That fan assembly is really spinning fast, and the bearings over time will give out.
Most likely, unless you can find something simple, you will need a new assembly.
Good luck! Steve Scott
