Speedometer problem 56 GMC

Noticed tht my speedometer needle on the GMC falls to 80 mph once you actually reach 55mph. this is the original 0-80 mph speedo and has worked flawlessly in the past. Any ideas ?

 

if the speedo cable core, the center of the speedo cable is exerting any pressure on the drive itself, it might cause trouble. or the cable is loose at the speedo, once again exerting side load pressure on the drive

 

That's what they do when they wear out. You can try making sure the cable core does not push against the speedo, you can also try adding a drop or two of light oil to the speedo where the cable goes on (between the rotating collar and the threaded housing). But it probably needs to be rebuilt.

 

I was going to suggest light oil like Squirrel said. I've had them do that from lack of lubricant. Dean

 

Thanks I will check the cable and try a little oil !

 

Squirrel, you da man ! that fixed it. I usually overthink things. I was thinkin somethin was screwed up in it

 

Well, something can still be screwed up. Those units were made to be serviced and it's never a bad idea to have it checked out.

 

I agree, how about giving us a how-to for working on them? I'm sure a lot of guys here would love to see how it's done, and seeing what's involved might convince them that it's something that should be done by a pro.

(in case you haven't noticed, we're into do-it-yourself stuff here, not so much for sending things out to get them fixed unless there's a good reason)