Here is what I suggest before you do anything else:
Remove the clip just behind the gear.
Slide the gear off.
Remove the flexible core.
(You do not need to remove the cable from the truck, just the core.)
Be careful as you remove it, it flops around and can get dirty real fast, and they are hard to clean.

Inspect the core for evidence of fraying or obvious wear. Shiny spots are OK, but frayed, broken or kinked spots are not.

Here is why I recommend you do this:
You gear shows evidence of stress. Although it seems quiet when being turned with a drill, there may be something binding the core anyway that the gear cannot overcome when in normal position. Or, the core may be broken but pushing against it with a drill may allow it to turn normally.

I really suggest you try it.

Re-installation is easy.
Slide the core up the cable.
If it doesn't seem to go in all the way, rotate the core slightly while pushing it in. It will eventually seat into the speedometer head (Notice the ends of the core are squared off.)
Re-affix the gear and fasten it with the clip.

Finally, if you look carefully around the flat surface of the gear on either end, there will be a 2 digit number with the gear tooth count on it. I have several 17 tooth gears, if you want one.

 

OK, going from newest post to oldest - Banjo - I have had the entire cable out and all looks good. No shiny spots, well lubed (again) and the R&R verifies the correct length with the slight rotation at the end to get it to drop into the driven gear. An interesting note is that with the head disconnected the cable is not being driven in fwd motion but the cable is being driven in reverse.

CDH - Using the calculations you provided I come up with the following:
Tires are 255-70SR15 so dia=29.055 and cir=91.28 inches
Differential code is 17 (no tag) for a 3:25 ratio
Trans casting (no tag) C8AR-7006 C usless except for it being a 68 three speed.
So I made several assumptions, the first being that 6 teeth on the output shaft would only put the tooth per minute in the 13,535 range and a 14 tooth driven gear would put me at 966 or 3.5% error. If it is a 7 tooth drive gear then 15791 teeth per minute would get me to 986 with a 16 tooth driven gear or 1.4% error. The second being that if it were a 6 tooth drive gear and my 17 tooth driven gear were installed I would have seen a 25% error (even with the bouncing).

 

Raleighdad -- we know you have a 17 tooth driven, right? Given what you are saying, that the speedo was not too far off when it did still work, would seem that you have a 7 tooth spiral DRIVE gear.

Your observation that it rotates backwards in reverse, but not forwards, seems to me to prove that your spiral DRIVE gear is shot.

Assuming you really want that speedo to work, you are going to have to replace the DRIVE gear. I'd try some of Barry's numbers or Inland Truck as I suggested.

http://www.inlandtruck.com/

 

Here's what I've got.

C3OZ 17285-C Drive gear, 7 tooth. It has a I D of 1.38 inches.R/hand pitch
C3DZ 17271-B Driven gear 17 tooth.

Because the tranny is a car 3 speed not listed in truck books, I'm taking a small guess for the drive gear. I don't have the 65-72 car books yet ( too expensive). The car and truck 3 speed trannys share basically the same interals, this gear should work.

Green sales Cinci, Oh. 1-800-543-4959 qty 3 in stock
Wesley Obs, Liberty, Ky 1-606-787-5293 qty 2 in stock
miller Obs Vestal, NY 1-800-546-4959 qty 1 in stock.

because your tranny is a transplant, you may have to pull the gear off the output shaft and measure the shaft diameter and see which way the gear teeth lean (right or left). I have several books to see what you have. These gears were used from the early 60's thru the mid 70's. Not a hard item to find,

Barry

 

Thanks for the info and part numbers. I had to put this on hold this weekend for an emergency radiator repair. I already had a pinhole leak and was hoping to get thru winter when I brushed the fan side this weekend and ripped open three cores. They were like paper so I pulled the radiator, clipped and soldered the cores and tank, and the adjacent cores kept failing from the heat of repair. Those things were sooooo thin! I ended up finding a '72 radiator in stock at a local parts house and I removed the mounting brackets and reversed them for a perfect fit. The '72 brackets were configured for grill mount and the '66 used rear or shroud-side mounts. The original radiator had certainly served beyond the call of duty.

 

OK, Back to the speedo... I finally received my replacement driven gear and voila, it still doesn't work (no big surprise here) so it will have to stay on hold till I convert to an AOD. Thanks again for all of the very useful information.

 

Looks like you're down to replacing the drive gear on the output shaft. Not a hard thing to do. You can do it right in the truck, probably in a couple of hours. Alot easier than doing the AOPD swap and can buy you some time until you do the swap. Because you have the 3 speed out of a 70 truck, I'm uncertain on the gear. They both are 7 tooth, but, they show 65-69 as one number and 70 and later is another. The correct way would be to remove the old one and get the number off it. It's a pain. I found a dozen places for the early gear and just one on the late. I don't remember if you posted a tranny tag number. They usually start with RAT-* or RAN-*. I can I D the year.

Barry