Mid-way through this thread
Lubricating the Smiths Speedometer Cable To Stop Needle Bounce I give the diagnostics to deal with a misbehaving speedo, the cable, the driven gear in the trans, etc. Even though mine is a Smiths and TKO, the basic troubleshooting is the same for a SW. It is not a terribly difficult task, and if your cable is broken, your local auto parts store will have one behind the counter. I thought for sure I had the part number for the cable, but I don't. I'm pretty sure though that it's just a straight Ford part. Just take your old one to the store with you.