The driving wheel of a bicycle or moped is fitted with radial spokes for ready detection of angular divergence between hub and rim under the action of torque. The angular position of the hub is detected by means of a perforated disk having openings which pass in front of a photoelectric sensor. A hoop attached to the rim is provided with rectangular notches which also pass in front of a photoelectric sensor. The signals produced by the sensors are sent to an electronic unit for computing the torque from the time interval by which the hoop lags with respect to the disk. In addition, detection of the passage of disk openings serves to calculate the speed which is displayed together with the power obtained by electronic multiplication of the torque by the speed.