How to Properly Lubricate Speedometer?

I can't conceive of any way grease on the cable could affect the accuracy. The cable turns the same rpm regardless.

When the speedo bore gets too worn, the spinning magnet can wobble. Worst-case, the magnet can grab the aluminum cup. That happened in my 65 Newport. Fortunately, the cable broke and I was able to bend the cup back. I sent the spinning magnet to a speed shop in Bakersfield and they re-bushed it. That was ~1992.