I have a 1972 Z with the typical "waving speedometer problem".  I tried cleaning and lubing the cable, but it was just too far gone.  The problem was getting worse and then the speedometer started reading about 15 mph low.  I replaced the cable and the waving problem went away immediately, but the speedo now reads too slow. 

 

I read a great post from a while back how a speedometer needle can get moved from its correct position.  Should I try removing the speedometer and resetting the needle position?  I know the speedo pinion gear in the transmission is correct for the read end of the car, so unless it has been damaged I don't think that is the problem.

 

Thanks and Happy Fathers Day!

 

 

Remove the speedometer and take the needle off. Put it back around 5-10 min (depending on what year speedo you have) below the pin it rests on and then gently pull it to the correct side of the pin. This should give a correct reading. Hook the cable up and take it for a test drive. You may need to do it a couple of times to get it 100% correct so dont put it all together before you have tested it.

 I know the speedo pinion gear in the transmission is correct for the read end of the car, so unless it has been damaged I don't think that is the problem.

 

Tire size has an effect also.  You can tell if it's an offset or a ratio problem by the numbers.  If it's the same number off over a wide range, like 3 mph at all speeds, then moelk's fix will work.  If it's off more the faster you go then it's a ratio problem and you'll need a different gear.  Might have to live with it since the gear ratio choices are limited.