My 67 gtx came with a 150mph speedometer. The belvedere's came with the same speedo, but a 120mph instead. What I am trying to figure out is if there is any physical difference between the 120 and 150 speedos. The magnets and the cups for each seem to be the same size. I tested both with a constant speed electric drill, and they seemed to read about the same speed. From what I can gather, these should interswap?