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1970 convertible automatic out for 50 miles trip...half way no surprises, way back ran her up to 80 ish then Speedometer went haywire, varying from 80-120. Now varys from 70-80 at approximately 65-70. Then does not go to zero at stop (30 mph) and at approximately 30 it's reading 50-70 and bouncing around...it definitely is not working correctly.
Is this a speedometer cable issue or speedometer gear issue, or is it a transmission speedometer gear issue? Help?
A bouncing speedo needle is usually a symptom of the cable needing to be lubricated. When I got my 79, this was an issue, but it was not nearly as bad as yours sounds....
I pulled the cable out, removed the inner portion from the sheathing, and greased it. Put it back together and re-installed, it solved my issue. The 30MPH offset you describe may indicate something else, but this should at least be a place to start.
I had the same thing on my '70 BB,had to have the speedo rebuilt...but lube the cable first. On the plus side it was the first time my dash had been taken apart, and I found a build sheet tucked up inside it.
Use powdered graphite. Pull the cable out and apply to cable. Also put some in the hole of the sheath before reinstalling the cable. That should do it. I know on my '71 there is a bracket to guide the cable properly where it comes out of the transmission to keep the cable from having too much of an angle. Check to see if that hasn't fallen off or is missing. You can see it in this picture.
You may have a speedo that is just died. I had something similar happen on my 65 Impala years ago. It suddenly pegged the needle and started screeching loudly while cruising down the highway. In fact it broke the needle in half when it pegged. Start with lubing the speedo cable with graphite, like others have said. If it doesn't help, you may need to pull the speedo out to test it using a drill. I have used just the cable part of the speedo pushed into the speedo then attached a drill to the other end of the cable to test speedos before. If the bouncing and high speed reading continues, you may need to replace the speedo.
Scott Liggett
Product Specialist
BluePrint Engines
Erratic Speedometer
