my new 76 speedo worked fine know its not accurate it will move but not smooth very jumpey. any ideas of problem thanks in advance

 

I have a 76 and have been through the speedo troubles which I have since resolved so I'm glad to help. When you say your new 76 speedo do you mean the speedo is new or you recently bought your car and it is new with the speedo that came in the car? How long did it work before the jumping started? Have any other changes or repairs been made to the car recently?

When I got my car the speedo was off by 10 mph so I started changing the driven gear which is the one the speedo cable connects to and got it within 5mph but above 70 it started bouncing around. Below 20 it's seems normal that they bounce.

Since then I've replaced my transmission and found the drive gear (gear on the output shaft that connects to the driven gear on the end of the speedo cable) was a different number of teeth from the trans that was in the car, so again I had to change the driven gear to a different number of teeth to slow down the speedo.

Right now I'm within 1 to 2 mph according to the gps on my phone but my tires are slightly shorter than stock so this is the difference. In addition there is no more jumping needle at high speeds, a slight delay in accuracy under heavy acceleration but it catches up within a second or two.

So you may have the wrong speedo gear or if the speedo is original it may need to be overhauled or replaced with a rebuilt unit.

 

Jumpy is most likely a cable issue. I recently put a generic (Autozone) cable on my car and it bounce around quite a bit (too long and forgot to lube). I have the correct one, but I'm waiting to do an engine/tranny swap so i don't do the work twice.

I'm now 5mph off at 25mph, but only 1-2 at 45-65mph

If you didn't do anything to your cable to cause this, you may just need to lubricate it.

 

sorry the cars new to me the car is all orginal. when i was test driving it it seemed to work fine aweek later im driving and noticed it was stuck on 40 than i slowed down it went down but not smoothly and not in sink with actual speed thanks