Tire pressure monitors (10) are incorporated in vehicles (12) having pneumatic tires (15) to continuously or periodically measure air pressure within each tire and to alert the driver should the pressure in any of the tires fall below a predetermined value. A passive transponder (22) is affixed within the drop center (41) of each wheel (17) and includes a pressure sensing switch (36). An RF transmitter (46) centrally located within the vehicle body interrogates each transponder, either serially or simultaneously. Each transponder, in turn, reflects a predetermined harmonic of the RF signal back to a receiver (48) as a function of the state (open or closed) of its associated pressure switch. The receiver then outputs a signal to a processor/display unit (50) to activate an alarm or indicating device (58).