Make sure the cable itself goes in far enough to engage the speedometer mechanism, then take the cable from the tranny and chuck it in a drill to see if the speedo needle moves. If no movement, get one at a swap meet or try e-bay. If the needle does move, put the cable back in the tranny, amking sure it is engaging the nylon gear. Leave the cable unhooked from the back of the speedo. Then go down the road and observe if the cable itself is moving - a passenger would be helpfull to avoid accidents. If cable still not moving, then the nylon/plastic gear or worse yet, the internal gear may need replacing.
The plastic gears can still be obtained, the internal gear may have to be replaced with a used one.
