The 1957 3100 truck I am helping with has speedometer issues. It has worked perfectly over the last year. Bear with me please. I think this may be a transmission issue. At very low speed, speedometer jumps, 0 - 60. once you reach speed, i.e. 50 mph, the speedo smooths out. First thought, it was a bad cable, although the installed cable was less than a year old. Replaced cable. Problem remained. Speedo was rebuilt by a very reputable company about a year ago. Took it back to them. They hooked it us to their testing rig, and the speedo functioned perfectly. 0 to 90 was perfect, repeatedly. The speedometer driven gear mounted in the tail housing was replaced about a year ago. it has a steel gear which does not appear to be worn and rotates smoothly. Now to my question. Could the speedometer drive gear, not the driven gear, be slipping on the main shaft? Is it keyed? Could tightening the nut at the end of the main shaft possibly correct this if it is not keyed? Before I drop the driveshaft I thought I would ask for other potential solutions. Nothing has been done to the transmission over the last year. No changes made, etc. The speedometer just started jumping. Thanks for the suggestions and guidance.
