A system is provided for use with various types of vehicles employing pneumatic tires to signal the operator that the tire pressure is beyond a preset range. A spring-loaded pressure responsive piston is adapted to reciprocate within a valve structure in response to changes in tire pressure. If the piston is displaced one way or other beyond the predetermined range, air is released to operate a whistle detected by a sensing system on the vehicle which, in turn, generates a signal for the operator.