Only a tach that is driven by an impulse counter which is mechanically driven , or a tach that is cable driven can perhaps be made somewhat accurate.
Most that are alt driven are no where near accurate.
Alt temp,alt load , belt age and load , pulley temperature all vary , as will the tach reading.
Best you can do with one of these is to use a photo tach at normal cruise and set the electric toy to match it.
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