Speedometer Dial Jumping

[Marconni]

Not sure if anybody has come across their speedometer jumping? when I reach over 40mph In my 1970 mgb the speedo like jumps back & fwd about an extra 5mph. Not had time to remove the speedo cable as yet, came across similar issue many years ago with a motor bike I used to ride, & that was a result of a broken tooth on the cable. Any ideas.

[Statler]

Initial thoughts are a frayed inner cable if it's ok up to a certain speed.
From what I've read the inner cable should be oiled to aid turning. Mine was full of the stuff as the seal on the gearbox speedo gear had failed. I left it hanging with the engine out and oil seeped out of it for months....

I had to pull the gear and replace the o-ring. Glad I was able to do it with the gearbox out as it looks like a difficult job in situ.
The plastic speedo gear wheel was Ok in mine, but that's something else to check along with the level of the oil in the gearbox as they are related.

Before you start into that though, it may be worth pulling the cable from the gearbox and turning the core to see if you feel any resistance. Once you work out the correct direction of turn, putting a drill on it and slowly run up the speed to see if the speedo gauge still jumps with it off the gearbox may help diagnose the frayed cable. Be careful with the end of the cable, it's moulded square to fit into the gear, don't crush it.
It's possible it could be the gauge itself and the above may help rule that in or out.

[Bumpa]

It can be too much oil or grease in the cable. The rotation of the cable seems to bring the oil upwards and if it gets into the instrument it may cause the fluctuations you see. There is no mechanical connection between the cable and the speedo needle. The needle moves because of a spinning magnet driven by the cable affecting the needle. The speedo needs to be fairly clean inside.

But as Statler said a frayed inner cable is the most likely cause.

[Vic Butler]

Before I replaced the speedo cable on my Land Rover, I removed the inner and sprayed it with Drylube silicone oil. I then replaced the inner wiping the top 6 inches and hung it up for 24 hours. Result, a steady needle, even on a Land Rover.