How easy is it to obtain drive/driven gears that work together and give you reasonable results?
I had an adapter made-to-order by the Speedometer Shop and it reads with almost no error. You give them a mile-marker mileage number - this way it takes into account your transmission, rear-end, and 16" tire diameter.
I'll do this again with the 1954 pickup that I am now restoring: 3.55 rear end, original Hydra-Matic transmission, and radial truck tires.
Jim Carter also sells these adapters and his price is quite reasonable (about $75).
