Device for measuring speed of a cycle having a frame, at least one wheel, a fastening axle about which the wheel is adapted to rotate, and a nut adapted to be threaded upon a threaded portion of the axle. The device includes a mechanism for detecting the rotation of the wheel, the device including a fixed sensor mounted on a portion of the cycle frame, adapted to detect the passing of an associated movable sensor element fixed for movement to the wheel, as well as a signal processor for processing the signal generated by the fixed sensor. At least one of, or both, the fixed sensor and the signal processor are housed in a casing fixed to the nut of the fastening axle for the wheel. The casing is fixed to an arm extending radially from the nut.