there used to be speedo adapters available, which were little boxes which were like 2" long x 1" x 1", and screwed to your trans and accepted the end of the speedo cable. I've seen them made out of bakelite or aluminum. within the box there are little nylon? spur gears, of various numbered teeth, to alter the final ratio in conjunction with whatever speedo ratio in the trans. My 76 dodge w200 came with one from the factory.
