Excuse some of my typo ramblings in the previous post. When I spoke about the nylon bushing on the tip of the cable at the speedo head, what I meant was a you begin drawing the cable from the casing, just under the squared tip you should see a nylon filigree bushing washer. Also, when you go about reinserting the cable back into the casing, be sure to liberally use a good, non-hardening cable lubricant.

Disconnect the cable casing from the speedo head and take a look at the nylon bushing at the tip of the cable. If it's present and appears intact then draw the cable from the casing while pinching it in between your index finger and thumb. You're looking at the cable for twisting, kinks and observe for excessive wear at the ends of the cable where they engage the speedo head and drive gear in the transmission. You should have a relatively squared tip at the head, as well a square boxed end at the driven end. If all appears correct, the next cheapest step is to yank the driven gear from the transmission. That gear should a nice twisting gear pattern with raised gear teeth. IF the gear seems to be beveled in the middle, showing very little teeth pattern left on the gear, there's your problem. I'd also recommend sticking your finger into the opening for the drive gear making sure it's not moving around on your output shaft. Lastly, in most cases, the reason your driven gear has suffered tooth annihilation is usually due to the output shaft bushing of the trans is toasted, wobbling the driveshaft yoke around. If none of these are problematic conditions you see, the last step would be the speedo head.