['68OrangeSunshine]

If you disconnected the speedo cable at both ends, you could pull the spiral flex rod from the Cab end and inspect it. If it's OK, then squirt some canned air to clear the tube. Before you re-insert the flex Rod, squirt puffs of Powdered Graphite into the tube. Then add more puffs of
the sticky, black powder on the sides of the Shaft as it goes back down the tube.
I had an original Speedometer jumping around above 60 before I lubed it with Graphite Powder. Now it bounces a lot less.
Beware of black smudge, however.