I think if you take the cable loose at the trans. you can pull out the center cable. If so, clean it real good and relube it. Don't use "white grease" I heard it will tunnel inside the outside cable and the inner cable won't get any lube. Check with a speedo shop for the correct grease.
You may also have a dry bushing inside the speedo. That's rebuild time.
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ACES #2637
TEAM CHEVELLE #69