I've got a 1971 240-Z that has a brand new speedo cable installed but I am getting an up/down jumping of the needle at almost any speed. The jump is about 5-8 mph so it's difficult to know how fast you are actually going, plus it's irritating. Is this an electrical or a mechanical issue? Is there a quick fix for it short of a speed rebuild?

I also have a speedo issue on my 1972 240-Z. The regular odometer records mileage accurately but the trip odometer gets stuck and actually stops recording mileage after a while after I zero it out. The point at which it stops working seems to change with each tank of gas, not a particular number. I'm guessing it may be a lubrication or cleaning issue. Any other ideas???

Try repositioning the cable as is comes out of the tranny and goes to the firewall, make it as smooth as possible. Also did you lube the cable before you installed it ? The cable is more or less a giant spring and it's snapping around inside the cable sheath due to a kink somewhere along the cable, that's what is causing the needle to jump around.

Either a kink or a poorly lubed sheath, but definitely a symptom of being stuck and "snapping" forward (that's why it's jumping).

It doesn't hurt to take out the whole cable, check it for straightness then lube it before re-inserting into the sheath. Also as Zforce mentioned, make sure the route from tranny to speedo is as smooth as possible, i.e. no sharp bends in that sheath.

FWIW

E