I would never use electronic instruments on a pre 1980 bike if they were not available at the time of manufacture. The joy of seeing a classic bike lies in it's authenticity. In Australia we have Manx Nortons in historic races sporting Scitsu electronic tachos. They serve a purpose but that is not what I go to historic races to see. The original Manx tacho costs an arm and a leg, but what is historic racing about ? Similarly with 1970s and earlier road bikes - why would you have one which is obviously a bitsa ? You might as well ride a chopper. My Seeley 850 is basically a 60s bike, the tacho is off a Suzuki RG250. It looks exactly like the tachos which were used on Italian racers in the 50s and 60s. It is cable-driven and the gearbox on the back of it is reversible by undoing two screws - and it is the correct ratio for the Commando drive. It even has the correct rubberised mount with the big 'O'-ring.