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We have a 67 convertible (6 cyl/3-spd on floor) much has been mostly redone but am having trouble with the speedometer which "jumps all over the place around 30mph". Is smooth below 30 and above 40. Have replaced the head - no difference. Removed the cable - 2 parts with a square metal box connected between the 2 cables - nothing on the box other than the 2 connectors. Think I need a replacement but am confused by the replacement cables listed - 1 upper and 1 lower listed which match the lengths of what I removed and then there are a couple single length ones. Not sure what to do.... Replace exactly what I removed (what would that metal box be for?)? Replace with a single length? or....? How/why we have this car is another story.  Thanks