A speed measuring device for a wheel hub assembly equipped with a rolling bearing. The measuring device having an encoder wheel mounted on a rotating ring of the bearing, a mechanical support disposed between the encoder wheel and the ring to cause the encoder wheel and the ring to be angularly fixed to one another, and a mechanical lock for axially locking the encoder wheel in a seat formed by the mechanical lock together with the mechanical support. The encoder wheel is equipped with a base structure made of plastic material mounted in a seat.

A charger incorporated in a vehicle wheel supporting rolling bearing unit detects an output voltage of an electric generator and calculates a rotation speed of a hub. When a rotation speed of the hub is in a low speed state where a sensor and a wireless communication device are operated by electric power supplied by a battery, a frequency of wireless communication performed between the wireless communication device and a calculator disposed on a vehicle body side is changed according to the rotation speed of the hub or a traveling speed of a vehicle.