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