Ford Repair: speedometer trouble, speedometer cable, instrument cluster

Question
i have a 1981 bronco full size custom with 5.0 302 all stock and it is 4wd. my speedometer isn't working correctly. while your driving the needle fluctuates largely between about 10-15 mph. the faster i go the less it fluctuates less. how would i go about solving this issue?

Answer
not real familiar with the year of this truck but, from the symptoms, it appears your speedometer cable is binding. the cable attaches into a gear on the trani and the goes in to your instrument cluster. it's usually 2 pieces. replace the first section coming off the transmission