Erratic Speedometer

[thumper56]

My 56 truck has a TH350 tranny and stock instrument cluster.

This combo has not had speedo as of yet. Just put in a new cable and did calculations for correct driven gear size in respect to differential, tire size/ rotation per mile, and drive gear. Gear I came up with is not a perfect match for the equations sum- but close.

Problem is now speedometer has come to life- but reads EXTREMELY erratically. As I just start to roll- it jumps to 40 mph then back to 10, up to 50 etc. No noises are heard, and all connections are secure.

Question is... is this indicative of a faulty speedometer gauge? I have never been inside one of these critters so understand they are a precision device but don't know how they tick.

Thanks

[mr48chev]

You did lube the cable when you put it in?

Is the housing routed smoothly without any real tight bends?

I've taken a lot of speedometers apart and cleaned them and reset the odometer but I'd be hard pressed to write a tutorial on how to do it.

[thumper56]

Good points. There are no sharp bends but I did not lube the cable as it slid and rotated super easy in the sheath- will lube it.

But what triggered you to clean your speedo? Was it noisy or operated erratically? Other than just plain not functioning, what are the indicators that service/ replacement should be done to the gauge itself?

Thanks