I have a custom vehicle that uses 1972 1600cc VW parts and has the cable driven speedometer in MPH. I was curious if there is a reliable product that I can use to convert from anolog to digital to allow for smaller gauges/custom clusters.
Electronic speedometers typically pickup a signal from an electromagnetic pickup which takes a signal from a magnet attached to the output of the final drive. It is possible to adapt certain gearboxes which provide a cable drive to use this type of pickup such as this or you could fit ABS rings and take a feed from the wheel speed sensor. Effectively, all you need is an electrical "pulse" and a speedometer capable of being calibrated to this signal.
One solution is to use a GPS Speedometer. Google it and you will find lots of options.