Analog tachs go way, way back to magneto days.
Today, the circuitry usually has some kind of transistor/IC circuit which gets a signal from the engine spark ignition and creates a "standard" width pulse for each spark.
The faster the engine runs, the more of these "standard" pulses are generated each second. A mechanical (analog) meter averages all the pulses over a fairly long time span to give a steady meter reading.
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an older design but the explanation is stillvalid.
You can build a circuit today with a couple of 555 IC's.
Speedy Jim
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