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1946 2 ton

The speedometer did not work.

I found that the speedometer cable was missing when I disconnected the housing. So I ordered a new cable and housing. Installed them and the speedometer still does not work.

I noticed that there is a thin rod that is connected from the 2 speed rear knob on the dash.

The straight rod has a bent end and it appears that should be in the end of an arm on the speedometer housing. But that rod goes through at least an inch past the bent end.

Now it appears that the rod should move the speedometer housing so the speedometer reads correct for each rear gear.

Should the rod not go past the bent end into the hole?

Should the speedometer housing position be at 12 o'clock?

It appears that then the rod would not go past the bent end.

You should at least see an indication it just might not be correct, with the cable disconnected from the speedometer do you see the cables center rotate when you are moving forward?
Some one can just watch the end while you drive or if you have junk laying around like I do you have an old speedo or a mechanical tach that you can test with.

This will check that you are getting drive from the rear.

If this looks OK, I always keep a short piece of speedo cable handy to chuck up on the drill for testing the speedometer itself.

Once you get a indication, then work on the rod for speed shift to show proper speed.

I know this and wish I had an old piece of speedometer cable around.

What I do not know is where the arm that pivots on the back of the speedometer that the rod for speed shift connects to. The end of that is connected to the speedometer head pivots, should it be fixed?

Should that arm be at 12, 3, 6, 9, o'clock position?

Should that arm on the back of the speedometer be able to pivot or fixed, meaning that end should not pivot?

http://chevy.oldcarmanualproject.com/parts/1929_54/362.htm

Vacuum or mechanical shift?

Vacuum start at 3

Vacuum.

I made a mechanism for my GMC from a vacuum advance,as per an article I saw on here someplace;it wasn't hard to do,and it shifts the speedometer just fine. I can't find a speedometer cable that works for mine;it's square drive at the speedometer,but old style round with a blade on the end that goes to the Brownie (Military?),unless someone makes an adapter for one end or the other. (I've seen adapters on eBay,but not being able to pick 'em up and look into the ends,I can't tell exactly what they do.) I asked the mechanic at a Truck Shop about it and he just stared at me like I was crazy.
Speed

This truck does not have a Brownie. And my son has the truck back down island again so I can not check things out for a while.

The Brownie is not a factor on your truck-I only mentioned it because it's keeping me from being able to check the operation my new vacuum speedometer shifter on the road. If you can look in the assembly manual,there's probably a picture of the OEM mechanism;it'll be easy to figure out how to make what you need from that.
Speed

I have a 1946 1.5 ton. GMC. JC sold me a speedo cable (metal) and it did not have the blade on the end. I sent it back and they sent me another one (JC I thinks works with a local shop) at no additional charge with the blade on the end.

Apparently those square cables come in sizes, 3 as I recall, so it might be an issue for some.