A radio frequency (RF) transponder (200) associated with a pneumatic tire and capable of measuring operating parameters such as temperature and pressure within the pneumatic tire, and transmitting data indicative of the measured parameters to an external reader/interrogator (106, 400). The transponder includes circuitry (226) for controlling windows of time (WT and WP) during which real-time temperature and pressure measurements are made, and for storing (236) calibration data, transponder ID number and the like, and for transmitting this information in a data stream (Figure 3C) to the reader/interrogator. An excessive temperature condition may also be sensed (MTMS 218) and included in the data steam. The circuitry of the transponder is preferably implemented on a single IC chip (204), using CMOS technology, with few components external to the IC chip. The transponder is preferably passive, deriving its operating power from an RF signal provided by the external reader/interrogator. Data (NT) indicative of temperature and data (NP) indicative of pressure are both transmitted to the reader/interrogator, along with calibration data. A calibration data stored by the transponder and transmitted in the data stream is a slope of NT/NP, or the 'ratioed' response of the temperature count divided by the pressure count, which is determined during calibration of the transponder.