Speedometer measurement is measured by reed sensors. The rotational speed (RPM's) of a wheel is transferred to a ring-magnet with multiple poles through the actuating shaft.

Electronic speedometers work in a completely different way. Small magnets attached to the car's rotating drive shaft sweep past tiny magnetic sensors either reed switches or Hall-effect sensors positioned nearby. Each time the magnets pass the sensors, they generate a brief pulse of electric current.

In general, it is unsafe to drive a vehicle with a broken wheel speed sensor. The car's computer will not be able to determine the speed that your vehicle is traveling, so it might apply the brakes at a faster rate than necessary or increase the engine power.