Speedometer repair.
 

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Its me again, working on reviving a Chevy G30 hi-cube box truck that sat for years. I am taking a different approach to this repair by asking for advice first, instead of when I get stuck, thanks in advance for any tips you may have.
I am trying to make my speedometer work, and hopefully the odometer as well. I tested the speedometer cable by turning it with a drill, the speedometer needle DID move. Am I correct to believe this means both the speedometer itself and the cable are good? If so, that means I move on to checking the drive gear and driven gear? (I have been watching videos, but as usual I only see part of what I need to know.)
So.There is the speedometer cable, it screws into a rectangle unit that plugs into the gear sleeve and the gear sleeve is on the transmission casing. (1) What is that box called, in case I have to do something with it? (2) DOES it just plug into the gear sleeve? As in, if I eventually have to remove that unit, use some muscle on pulling it free because I am not breaking something this time?
It looks like the sleeve comes out easy, but is there anything tricky I need to know or make note of as it comes off? (I've been burned before, recently.)
At this point, I'm looking to see if the driven gear shaft is broken, correct? This seems to be a common enough problem. If it's broken I should just find the shaft and gear just lying inside the cavity?
The Videos I have been able to find are vague about the placement of the driven gear. Does the shaft fit into the backside of the gear sleeve? And does it then just fit into the transmission cavity and fall perfectly into place, meshing with the drive gear? Or am I about to get into another head-scratching mess?
That's all I can think of in advance of taking things apart, is there anything you think I should know so I don't get in over my head again?

Thank you, Chuck
1984 Chevy G30 Hi-cube
350 gas engine
Th400 transmission

Speedo calibrator?,square drive,both sides,easy off and on

While I am waiting for the driven gear to arrive I have a question. I spun the speedometer cable with a drill. The cable turns freely and the needle on the gauge moves. So what caused the shaft of the driven gear to twist off? Does that just happen with use? Is there something else I should be looking at before installing the new gear?

Thank you, Chuck
1984 Chevy G30 Hi-cube
350 gas engine
Th400 transmission

I would think,cold weather,resistance in cable,binding,stiff,old lube,go for a new cable

Lube the speedometer cable before re installing.