See the complete circuit in my site to study other sections. The motor DC out is filtered and scaled. Then the clean safe dc is fed to this VCO based on a generic application note design.
The pulses can be used to advance digital counter or mechanical counter using a solenoid driver transistor circuit. The Voltage Vin can be further scaled and fed to a Digital Panel meter to read the rate.
VCO - Voltage Controlled Oscillator using Opamps. LM339 is a quad Comparator.
The complete PDF circuit and other such circuits at my Mixed Circuits Analog with Digital