A pneumatic tire monitoring system is disclosed which provides a pressure sensor/transponder unit for each pneumatic tire of a vehicle to monitor tire pressure. A coded signal transmitted from each transponder unit is received by a multi-purpose receiver which may provide information to the vehicle operator or to other vehicle systems and subsystems. Each transponder unit is characterized by ultra low power consumption to extend the useful battery life. The transponder units may be mounted within the vehicle tires, or externally mounted. Impedance matching circuitry allows the use of the wheel rims with the transmitting antenna so as to avoid doppler shift induced by the rotating antenna and frequency drift induced by capacitive coupling.