A device for indicating when the air within a tire is above or below an optimum pressure range, comprising a housing having a bore therein in which a piston is movable. The piston is spring biased toward one end of the bore and is normally held in a medial sealing position by air pressure from within the tire. In such sealing position, a sealing ring on the piston engages a sealing section of the bore to prevent movement of air along the bore; when the air pressure exceeds or falls below the optimum range, the sealing ring moves over a relieved portion of the bore and allows air to move along the latter to actuate a whistle carried by the piston. An electronic device senses the whistle and signals the operator.