Excessively high or low tire pressure is detected automatically with a warning signal generated for the operator. A device is attached to each wheel and includes a spring-loaded piston mounted for reciprocation in a cylinder formed in the device. The cylinder is provided with a porting arrangement which opens only at certain pressure limits. When the ports are opened a sonic or ultrasonic signal is generated and detected by sensors located in proximity to each wheel. The signals detected by the sensors are processed and utilized to actuate a warning signal that provides a visual and/or audio output for the operator. Means are provided for discriminating between background noise and the generated signal as well as to discriminate as to the location of the affected tire. In a modification of the invention, a reserve chamber is integrated into the body of the wheel and is adapted to replenish automatically air lost from the tire.