You might want to check the speedo cable for lubricant. Yes they need to be greased (been there, had that happen). Disconnect it from the speedometer, pull it out, grease the snot out of it and then reinsert it in the cable housing. You can also remove the cable housing connector at the transmission to make sure you get the cable all the way in. Then hook it back up to the speedometer and give it a try. BTW, while your under the car disconnecting the cable housing you can pull the speedo gear and count the number of teeth on it
If it wasnt made for this car, a hammer and screwdriver will usually adapt it to fit.
2004 Chevy Avalanche------Mine
1995 Chevy Camaro---------The Wifes
1965 "C" Code Mustang coupe---------The sons
1947 Beech Bonanza--------Family vehicle