Patent ID: 8297117

Claim:
An apparatus for detecting a decrease in an air pressure of a tire attached to a vehicle, comprising: a rotation speed detection means for periodically detecting rotation speeds of tires of the respective wheels of the vehicle; an acceleration calculation means for calculating, based on rotation speed information obtained from the rotation speed detection means, accelerations of the respective tires; an acceleration variation calculation means for calculating, based on the accelerations of the respective tires calculated by the acceleration calculation means, acceleration variations of the respective tires; a comparison means for comparing the acceleration variations of the respective tires calculated by the acceleration variation calculation means; a presumption means for presuming, when a comparison result by the comparison means shows that an acceleration variation of a tire is larger than acceleration variations of the other tires, a pressure decrease of the tire; an examination means for identifying, based on the tire rotation speed information of the respective wheels, a tire having a rotation speed that is higher than the rotation speeds of the other tires or that is lower than a predetermined criterion; and a determination means for determining, when a tire identified by the presumption means is identical with a tire identified by the examination means, a pressure decrease of the tire, wherein the predetermined criterion defines a degree at which, when four tires have a normal internal pressure and one of the four tires is fitted with a snow chain, the tire has a lower rotation speed than the rotation speeds of the other tires, the examination means is further configured to identify, based on the tire rotation speed information of the respective wheels, a tire having a rotation speed lower than the rotation speeds of the other tires, and the determination means is configured to determine that a tire has a decreased air pressure when the tire identified by the presumption means is identical with the slowest tire identified by the examination means and when the slowest tire is positioned at a driven wheel.