[Richard]

So...........this is my .02.

Buy electronic speedo and tach. Install the speedo sensor/sender on the driveshaft. Install the tach sender/sensor circuit on the alternator.

You guys are dealing with mechanical units. The unit is going to behave wonky if the spinning cable has any stutter to it. The chances of a corroded cable or housing in a coach of that age is high.

If you do pull the cable and get it all cleaned and greased, you still may have problems with the gears in the readout units. You will faint when you see what it costs to repair the old mechanical units. More than a new electronic unit.

And on the 6V92, the tach cable is driven by a gear unit that fits on the top of the engine much like a distributor. Ask @chockwald if he ever found a replacement.