I put a turbodiesel engine + transmission off a 83 300SD in my 75 300D. As a result, my original speedo does not talk to the transmission sending unit. I do have a speedo off that 300SD. Heck I also have a complete instrument cluster. But, what would be the best: make my car use the electrical sending unit or swap the end of the transmission with that of one with a mechanical sending unit? I just want the least ghetto route in the end of the day. :D