There are a lot of types of tachos for classics.

Mechanical
Electronic current sensing (eg Smiths RVI)
Electronic voltage sensing (eg Smiths RVC)

The mechanical type picks up the motion from a rotating cable connected to the engine.
The cable drives a rotating magnet that drives a spring loaded place of iron connected to the needle.
For example, MGAs Bugeyes and early MGBs used that type.
Triumph used this on spitfires, gt6, TRs Vitesses etc. up to mid seventies.
Alfa romeo and Lancia also used mechanical ones on early Spiders, GTs, Fulvias etc.
Mechanical tachometers are prone to…. mechanical failures.
Internal faults are frequents, the rotating cable is broken frequently.
Also the gears that drives the cable are prone to brake (for example the triumph spitfire’s
gear at the base of the distributor, that’s no longer avaiable).
Finally, as the internal parts of the mechanical tacho wear out, it become less accurate.

Early electric tachos were current sensing. Smiths for example call them RVI type.
Many were used until seventies.
A sort of simple input transformer serves to chatch the signal from the ignition
(the loop of wire in the back of the tachometer) because at the time the input impedance of circuit was low.
This leads to a series of problems because the input transformer is susceptible of the architecture of the full ignition circuit.
Changes of coil type, points, ELECTRONIC IGNITION conversion
may take to false or no readings.

Later electronic tachos were tension sensing and are many more reliable. Smiths call them RVC type.
They may be however affected by aging problems like losing of adjustment, faulty condensers or inaccurate resistors that may lead to wrong readings or malfunction.

uRevCo is a solution to all and in future I’ll explain why….