I would start and see if the cable is connected at both ends and is not rusted through anywhere. Hook a drill up to the trans end of the cable an spin it and see if the speedometer runs. This will tell if it is a cable, speedo problem or the gear in the trans if it is the trans gear when you go to get one tell the your gear ratio and trans type. Alot of times the cable breaks and the speedo will not move when the drill is run(you need a reversible drill I think).If it does not run disconnect from the back of speedo and run the drill on the trans end and see if the cable turns. If it turns speedo problem if it doesn't cable problem. Hope this helps
good luck on your trip.
