Actually, at higher rpms, the period of the CMP waveform gets shorter. Usual practice is to use a re-triggerable monostable multivibrator whose period is set to the critical rpm. If the CMP period retriggers the multi before it times out, the multi output is always high. If the multi times out, that means the CMP period is below the critical rpm.
A second multi (with a long period) edge-triggered by the first will produce a logic signal which is low if the rpm is too high (i. e. it stops being triggered).
Here is an LTSpice sim consisting of two 555s. R1 controls the critical RPM as shown in the table in the schematic The Led lights if the RPM is above the critical RPM.
I assumed that there are two pulses per crank revolution.