Your cable is fine then. If the odometer is working correctly, then something has gone wrong with the speedo.
I'm not sure what gauges you have, but you can put your hand under the dash to disconnect the cable from the speedo. It's a clip that you have to squeeze before taking off. From there take off the dash light panel, then you have to remove the switch panel, then the gauge bezel. This is not the easiest thing in the world to do, so use a large flat blade screw driver to maneuver the switch panel out of there. Once the gauge bezel is off, you can pull the gauges out, but you will soon find the wiring is pretty tight. You have to put your hand behind there and remove the wiring and turn signal indicators. Just remember where each wire/light bulb goes.
From there, you can remove the speedo from the cluster and send it out for repair.
Some find it easier to drop the steering column first. There is a bracket under the dash with a nut on each side that requires a deep socket to remove.
DISCONNECT the battery before attempt any this! Don't ask how I know.
