Abstract: A tamper-proof odometer of the type having digit wheels mounted for rotation about a shaft by gears on gear plates separating the wheels. At least one wheel is formed, for example, by scoring so as to break when an attempt is made to pry that wheel from an adjacent wheel to reset the odometer. The shaft is grooved and end caps are fixedly attached to the shaft ends so that the shaft will break if an attempt is made to remove the caps by drilling or sawing. The caps have different cross-sections so that the shaft can be engaged in a frame in only one orientation.