The Stovebolt Site Forums The Shop Area Driveline 1957 3100 4 speed speedo drive gear

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.

According to the 1955.2 Series Truck Shop Manual the speedometer drive gear is pressed onto the output shaft u-joint forward yoke. As there is no contradictory information in the 1956 Supplement, I think it's likely that your friends should also be press fit and tightening the yoke securing nut will have no effect if the drive gear is loose/moving on the yoke. A double check of the Master Parts books shows no key for the gear listed.

It is possible that the drive gear could be moving and causing the erratic speedometer function. The only way to know is to drop the drive shaft and remove the yoke to inspect. It's possible that someone could have improperly reinstalled the drive gear to the yoke. The Shop Manual indicates to be sure that the drive gear is installed with the beveled edge towards the front of the transmission. If it was installed backwards it could be improperly seated.

Let us know how you find things when you get into it.

Excellent explanation! I will pull the yoke and respond accordingly.

Thanks!!!!