DTC B2557: Vehicle Speed: Description
WARNING: This page is about a different car, the 2008 Infiniti EX35. 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. 1 signal is transmitted by the "unified meter and A/C amp." Another signal is transmitted by "ABS actuator and electric unit (control unit)". BCM compares both signals to detect the vehicle speed.