Thank you for the answers. It helps me a lot. I don't really understand where to put the capacitor and the resistor for the low pass filter. Is this correct:
I need to get the input signal between 120 Hz (17.5 km/h) and 1.2 kHz (175 km/h). Therefore, I think a cutoff frequency of 1.59 kHz would be correct, so I have to use a 10 kΩ resistor and a 10 nF capacitor or a 1kΩ resistor and a 100 nF capacitor.
Is it a good idea to use a LM2596 DC-DC converter to provide 5 V instead feeding 13.8 V from the Battery/Alternator to VIN?
I have realized that speedometer signal amplitude is 12 V, not 4.5 V.