There are two types of tire pressure monitoring systems (TPMS) in the world. One is an indirect TPMS based on the wheel speed signal data from ABS/ESP sensors. The other is a direct system based on a tire pressure sensor. The indirect TPMS systems for the most part have been unable to detect a 4-wheel simultaneous deflation condition (i.e. diffusion deflation) because the system detects the deflation by comparing the 4 wheel speeds with each other. In this paper, we discuss a method to detect diffusion deflation by using only the wheel speed signals.