The sensor screws into the Speedometer cable output on the transmission, sends pure sinewave back, increasing frequency and amplitude as speed increases. Initially at the equivalent of 10MPH/15KPH, the signal is roughly 20Hz and 50mV, but can be as much as a few volts at full speed. Op-amp is designed to go into clipping, has a benefit of making a nice square wave for an MCU to pulsecount/pulselength (speed/odometer).
Optional: diode and capacitor on the output that forms a low pass filter making it easier to filter out noise better without implementing a low-pass filter in software.
