Perhaps not so mysterious a design.  Your tachometer uses a pulse per unit
time to voltage conversion no digital stuff here nor D-A conversion.  So if
the transfer function of pulses per unit time  to voltage is known and the
desired range of ppt is known then the range of voltage is known.  A
comparator circuit can give you a measure of whether the voltage is within
the specified range.  If below then you have a signal that indicates engine
speed too low.  If above then engine speed too high.  Detailed design left
to David B or those with that sort of "bent".

Pensioner

Tau Beta Pi (bent), Eta Kappa Nu, Phi Kappa Phi in EE/CS,  O so long ago.