I have replaced the low ratio diff on my 1954 Devon pickup with a later higher ratio Somerset saloon item. The Speedo now of course reads slow and The miles driven are short when checked against my satnav.
If I can get hold of a Somerset speedo, are the internal parts interchangeable with my one?
I need to keep my original Speedo because it has the fuel gauge built in.
I think that as the speedos are of a different design. It would be best to get your original speedo refurbished and re calibrated to to match the new differential you have installed.
See the details on tis link. As the article says it is not a simple as we might think.