An electrically assisted cycle kit (50) for fitting to a pedal cycle (2) comprises an electric motor (58) for assisting pedal cycle motion, a battery (52) for powering the motor (58), and a controller (60) for controlling the drive of the motor (58). A gear tooth sensor (65) provides a gear motion signal indicative of the speed/acceleration of motion of gear teeth (94) and valleys (95) of a gear sprocket (32) of the cycle. The controller controls the drive of the motor (58) in dependence on the gear motion signal.