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Discussion Starter #1
I have a 1966 Plymouth Belvedere ll 4 door sedan with the 225 slant 6 engine. My speedometer needle bounces. I have replaced the cable but what else do I have to do? Does it need to be lubricated, gearing replaced? Also I just installed a new fuel sending unit but I don't think the gas gauge in my instrument cluster is correct. Can it be needing lubrication like the speedo cable or is it something else? Thanks in advance for any insight.
 

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I wound start by lubricating the speedometer cable. But the problem could be the speedometer head.

The gas gauge is provided a variable ground via the sending unit. Make sure there is a good ground at the tank for the sending unit.
 

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Think of the speedometer cable as a long torsion bar. If it is binding inside of the sheath, it will bind-release-bind-release, etc. This causes the bouncing needle.
When installing a new cable, avoid any sharp bends that may exacerbate binding.
Pull the cable core from the sheath and grease it as you roll it back into the sheath. If it sticks going in, pull it back out a couple of inches and then continue pushing it in while keeping it turning.



The fuel tank must have a good ground, or the gauge may read low or empty. There was a ground strap for the tubing that rotted out or disappeared. It must be there. The tank straps may not offer a good ground.
If the temperature gauge is also inaccurate, then the voltage limiter mounted on the cluster circuit board that feeds both gauges may have issues.

 
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Discussion Starter #4
Thanks, fellows. Appreciate the feedback.
 

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Where the cable mounts to the speedo, there should be a barrel of sorts. Check that it’s not “out of round”. Grab a socket set and find one that fits to see if it became oblong or dented. This would cause drag at the needle and make it bounce. Fitting an appropriate sized socket will true it up. Add lube and I hope this helps
 
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