It sounds like your speedometer cable is in serious need of lubrication. If you have cruise control, the transducer may need replacing. When it goes bad it makes the speedometer very erratic.

One or both of those is likely your problem.

I agree w/Gunslinger on the lubrication problem. If the noise is inside the cabin, it's more than likely you need to lube the cable. Disconnect the cable from the speedo head, and work it down to where you can get to it under the dash. Then pull the inner cable (carefully!) out of the housing. I would recommend disconnecting the cable @ the trans, and blowing thru it from the top if you have access to air compressor. If not, you could use some brake cleaner & spray it down the cable housing from the top.(You're trying to clean the inside of the housing.) Then clean the inner cable w/ brake cleaner also. Then you can soak the inner cable in clean motor oil or trans fluid, and reinstall it into the housing after draining off the excess fluid. Reconnect both ends and give it a try. That should take care of the noise and the fluctuations. If you do have C.Control, do this to both of the cables. Then if the problem is still there, then you will most likely have to have the transducer repaired or replaced. Good Luck!! btw. Some people may recommend using a dry-film lubricant such as a graphite-based product. That would be OK, too.

I know this may be a bit hard to believe, but I use wheel bearing grease on the speedo cables.  Just a thin coat the entire length of the cable.  It works very well.  I have done many, many over my years in the shop.  Do make sure it is clean first.kstyer38396.7940972222

i also use wheel bearing grease..  ive got one of those little cable lubricators though. makes it easy..

 

 

I have a jumping speedo at 10mph and less. The local garage said it is probably a flat spot on the gear. They lubed it, but it still chirps and jumps.
If I have to replace it, I'll get one that reads up to 120 or 140, instead of 85.

My speedo was having similar problems.  I think if you are going to the trouble of lubricating the cable you might as well replace it.  That's what I did.  Cables are very inexpensive.

I have two "77"'s both having speedometer problems.  One has stopped working altogether and the other has bounce and has had it for many years.  I have read a number of ways to dissassemble the cables ( both have cruise control ) not to sure on which is simpler or most effective.  I would appreciate any advice on this matter.  I believe the one with bounce needs a transducer.  where do I find that besides on the cruise control unit ?

 

                          Thank You,

                             Russell

The transducer,on my 78 at least ,is mounted on the left inner fender,below the master cylinder,and just behind the windshield washer tank.it is a sort of round unit with a flat side that faces the front of the car.it has a couple of vacuum lines plugged into it ,and 2 speedometer cables plugged into the other facing the firewall.and it can be a real bear to get out  

The transducer is a pain to replace if you have big hands like I do. The master cylinder is in the way and working around it leaves big hands a bit on the scarred side if you're not careful. That being said, to r & r the transducer isn't that tough...it's just a tight area to work in.