Replacing the Datsun 240Z speedometer cable is a fast-and-easy job, and it is the most likely cure for the “jumping speedometer needle” at low speeds.
I did carefully disassemble and oil the original speedometer cable, but I still had the “jumping needle” at low speed. Upon more careful inspection there was a slight kink in the inner speedometer cable and some places where the protective outer coating of the cable was gone.
I purchased a ‘clone’ speedometer cable from Black Dragon Automotive. The price was great.
The Black Dragon speedo cable is smaller in diameter and feels less ‘robust’ in construction than the stock cable, but for the price, I figure I can replace it again in 20 years if I have to.
My stock speedo cable had a strain-relief spring wound around the lower part, at the transmission. I carefully and slowly ‘unwound’ it from the old cable, cleaned it with my wire wheel and zinc plated it, then carefully re-wound it onto the new cable.
Routing the Speedometer cable
Here is the new speedo cable at the transmission. The cable curves in a nice gentle arc, thanks to the spring.
The new cable also does not include the firewall grommet/bushing, so I cut the existing grommet off of the stock cable (see the red arrow) and slipped it over the new cable. It was easiest to start with the grommet inside the cabin and push it out toward the engine. Getting under the brake booster and clutch master cylinder was not possible.
240Z Speedo cable & grommet at the inside firewall showing cut
See also:
Replacing Speedometer Pinion seals (at transmission). Where the speedometer cable attaches to the transmission is a common place for an oil leak. The fix is easy and inexpensive. I suggest just doing it while you are here!