68 GTS Speedometer doesn't work

Take the speedometer cable & case out of the vehicle. Determine which end the cable will come out of, only works one way IIRC. Clean the cable with carb or brake cleaner. Clean the cable housing the same way & let them dry completely. Soak the cable in clean 20 - 30 weight oil & reinsert. Check the speedometer gear for tooth count & make sure the insert is aligned correctly & reinstall the cable & casing.