cable driven Tachometer and speedometer

1 Make sure the cable has no sharp bends in it.
2 make sure outer sheath is not melted, kinked or crushed.
3 remove inner drive cable and check for kinks or binding.
4 clean and remove any rust or old grease from inner cable.
5 clean and blow out outer casing and then lube and reinstall inner cable.
This should get you to where you want to be if the issue is in the cable.

 

^^^^^^^What he said ^^^^^^^

 

There were also two different gear ratio tachs made back then. One is a 1.5:1 and the other is 2.00:1 I believe. It can be the difference of 1000 rpm.
One was for Detroits and the other was Cummins/ Cat.
Someone put an AC Delco tach for a Detroit in one of my Cummins powered trucks at some point and it was way off when I first got it running again. Had it screaming at probly 2400 when it read 2100.

Stewart-Warner still makes both ratios in the old classic “deluxe” line.

 

How do you know what the engine speed is.

Deal with this on some cars and my jeep which has a military mechanical tach installed, I removed the cable and cleaned the casing with a mild cleaner then I lubed the cable and casing with ac delco general lube.

 

I have had the cable end wear and jump in the socket. This was on a 359. The cable end was about an inch long and square looking at it from the end. As I recall I was able to purchase a new inner cable. This was 30 some years ago, but it's worth looking at both drive sockets, on tach and engine. The symptoms you describe were the same that I had.

You may be able to convert to electric fairly easy if the flywheel housing has a plug with which to add a tach sensor.

Edit to add: Like Ridgeline mentioned, clean cable will help. I believe I used to use dry graphite.

 

Speedometer Cable and Housing,Custom Made Tachometer and Speedometer Cables and Casing parts and Supplies.

These guys can set you up. They custom make any cable you need. They make the reduction gear boxes to calibrate the speedos to rear gear ratio or tire diameter changes as well.