I have a new d100 project with a 318/727. When I got the car the gauges were out of the car and the speedo needle was broken. Last week I bought a new, nice gauge cluster and was told that everything worked.
Once I installed the gauges, the speedo bounced and made popping noises so I replaced the cable with a new one from eBay.
After installing the new cable into my new cluster the speedo still popped and the needle still bounced. After about 2 minutes of driving that needle broke off as well.
So that's two gauge sets and two speedo cables both resulting in broken speedo needles.
Anyone have any ideas? Could it be the speedo gear in the trans? Truck has 3.91's and I have no way of knowing what's original or not. Any help is greatly appreciated!

jack up the truck as high as you can on the drivers side to minimize the fluid loss & pull the adapter/speedo gear for a look see for any damage & first note the clocking on the adapter to see if it matches the teeth count on the speedo gear when you pull it. holler back. EDIT even further before this, you might put a piece of the speedo inner cable in the adapter & start it & see if the cable turns smoothly without jerking (grab it with a glove to put some pressure on it) and you might stick the cable in the speedo housing if the other one still has the needle intact & spin it with your drill & see what happens. MORE EDIT & iirc you spin the drill CCW

Thanks Robert! I'll give it a shot as soon as I get a day off!

What's that? (I've heard of it). & you got a another outer sheath/inner cable or just a new inner cable? what I'm getting at is, the outer sheath did not appear to be bent/kinked/damaged anywhere along its length? Holler when you can, we ain't going anywhere.

I replaced the whole thing. Cable and sheath. I drove the truck for a minute with the gauges out and the old cable was definitely spinning inside the sheath.

Did you lube the cable? Was it a NOS cable?

Sure sounds like the clock spring in the speedometer itself. I know you were told it worked fine, but I'm still suspicious.

I broke 2 speedometer needles in my Challenger before figuring out it was the speedometer itself. $200 later, they replaced a bushing, and all is well.

Well, I’m looking for my 3rd speedometer now. I guess I’ll find out shortly....

check the speedometer gear. maybe it's installed improperly?

The gear installed improperly won't usually make needle jump wildly and you'll get leaking from trans where gear installs, and the plastic gear will show signs of wear or chewing.