56 truck speedo ,,

[JumboAMONGus]

hello , i have a 1956 chevy 3100 with original 3 speed , and rear end , the transmission was rebuilt years ago , works fine ,, but the speedometer only works in 1st and 2nd gear , once you put it in 3rd it drops to zero no matter how fast you are going ? the speedo gears in tranny are fine also is there something in the transmission shaft that engages when shifted to 3rd any info or a diagram , my manual does not show thank you ,

[_Ogre]

that is a strange problem jumbo. usually the speedo works or it doesn't.
i'd suspect that the plastic gear in the trans is worn enough that it disengages at higher speeds of the tail shaft
the teeth may look fine but if the shaft is worn enough it could disengage at higher drive shaft speeds.
you can check the speedo using a variable speed drill on the cable, you should see a smooth full speed range.
i'd try the drill test first and then move on to the drive gear

[1project2many]

If you shift into third at any speed, the speedo stops working? If you shift back out of third does the speedo start workign again?

Check the end of the cable at the gear, and the gear. The cable may have broken and a small amount is engaged in the gear for a bit then comes out as speed increases. Or the end of the cable may be rounded, or the inside of the gear may be rounded.