Tire pressure monitoring system and pressure monitoring unit

A tire pressure monitoring system for a vehicle in which a number of mounting wheels which is greater than a total number of running wheels to be mounted on the vehicle, are prepared, and some of the prepared mounting wheels are selected and mounted on the vehicle as the running wheels, the system comprising:

• tire pressure sensor units are respectively associated with the prepared mounting wheels, each of the tire pressure sensor units comprising a pressure sensor that detects a tire pressure, and a centrifugal force sensor that detects centrifugal force generated by rotation of a tire, and transmits, when a detected value by the centrifugal force sensor becomes equal to or greater than a predetermined value, tire-pressure data including a detected value by the pressure sensor and an identification code uniquely allocated to the tire pressure sensor unit to a pressure monitoring unit provided in the vehicle at a higher frequency than a frequency used when the detected value by the centrifugal force sensor is less than the predetermined value;
• the pressure monitoring unit compares the detected value by the pressure sensor in received tire-pressure data with a predetermined determination threshold when an identification code in the received tire-pressure data coincides with an identification code of a tire pressure sensor unit associated with one of the running wheels of the vehicle to monitor reduction of a tire pressure, wherein
• the pressure monitoring unit stores, for each of the prepared mounting wheels, the identification code of the associated tire pressure sensor unit and a set tire pressure value as a pair, specifies the identification codes of the selected mounting wheels as identification codes of the tire pressure sensor units associated with the running wheels of the vehicle, and when an identification code in tire-pressure data transmitted to the pressure monitoring unit coincides with one of the specified identification codes, checks a tire pressure with a set tire pressure value stored as a pair with the coinciding identification code as the predetermined determination threshold, and
• when identification codes in tire-pressure data transmitted to the pressure monitoring unit within a predetermined time while the vehicle is traveling at a speed equal to or greater than a predetermined vehicle speed coincide with identification codes stored as identification codes of the prepared mounting wheels, and when a number of coinciding identification codes is equal to a number of running wheels of the vehicle, excluding any received identification code which is a duplicate of another received identification code, and when at least one of the coinciding identification codes is an identification code which is different from the specified identification codes, the pressure monitoring unit automatically replaces the specified identification codes with the coinciding identification codes and newly specifies the coinciding identification codes as identification codes of the tire pressure sensor units associated with the running wheels of the vehicle.