well, im not sure what sort of speedo youre using and what difference that will make, but fcp groton and ipd sell the little plastic gears that can fix some OD/speedo problems that occur at the dash.. maye open yours up and see if a little gear is stripped, or has lost a tooth or two.. as far as i know, any model that early def. has a cable driven speedo and OD, unless yours has been modified with a separate electronic pickup for the speedo..

if it's not the gear inside, perhaps your gauge is faulty.. im sure is does have some kind of internal pickup/coil setup for needle movement.