My speedomater is not working (I own an 1989 Camaro RS, crate 350, overdrive tranny, not sure what else was modifiied like the rear end, too, when I bought the car last summer). I brought it to a shop who said it was a gear (there are two, this is, luckily, the simpler one) and he ordered and put it in. The speedo worked then quit. He inspected it and found out part of the gear was stripped; yes, he checked and the speedo cable is fine so he told me he is doing some research to see what the problem is. I figured this is the perfect place to ask if anyone here has some suggestions. Nice to know what speed you are going - not to mention that your odometer works!

 

Could be the drive gear (on the output shaft) is hacked up, and trashing the new driven gear.
You'd have to pull the driven speedo gear and housing, and peek inside with a flashlight to see if any teeth look chewed up.

 

Thanks, passed that along; let you know if this is what it is!