Abstract: The wheels at opposite ends of an axle - and which may be twin or tandem wheels -- each have a pressure switch and a tank circuit associated therewith, rotating with the wheel. The tank circuits includes a normally closed pressure-sensitive switch which, when the pressure in the respective tire drops below a certain value, opens the tank circuit. The coil of the tank circuit is coupled with a transducer coil connected to a blocking oscillator which, upon passage of the tank circuit, provides an output pulse. The frequency or recurrence rate of the output pulses from opposite ends of the wheels at an axle are compared in an integrating comparator and, if the frequencies differ after a predetermined integration time, provide an alarm signal. The integrator or a digital counter which should have a number of count stages, only the last one providing an alarm signal to allow for variation in frequency with normal tire pressure when the vehicle rounds a curve.