Reading car speed sensor (SDV)
A vehicle speed sensor works by monitoring the passing of a toothed wheel or some other source of magnetic interference on the transaxle of your car (though the exact location and mechanism may depend on your model of car.)
Because of this, it won't put out an analog signal but a PWM with varying frequency depending on how rapidly the shaft is rotating (and thus how fast your vehicle is moving). Without filtering, you likely won't be able to pick it up as an analog voltage, though the Arduino should be able to measure the frequency of the PWM and allow you to determine the vehicle speed with some relatively simple calculations. This appears to be what both of the above circuits are doing.