B2557 Vehicle Speed: Description
WARNING: This page is about a different car, the 2009 Nissan Maxima. However, it is still accessible from the selected car via links, so may be relevant.
BCM receives the 2 vehicle speed signals via CAN communication. One signal is transmitted by the "unified meter". Another signal is transmitted by "ABS actuator and electric unit (control unit)". BCM compares both signals to detect the vehicle speed.