Basic vehicle performance, such as Safety, Comfort and Economy, are by dependent on tire performance, and it is the air pressure in the tire which assures this performance. However, tire air has a tendency to leak naturally, making it necessary to check them periodically. Since a deterioration in vehicle performance resulting from a drop on tire air pressure can not be directly felt by the driver, the number of people maintaining their tires sufficiently is relatively few. There have been many tire pressure warning devices developed which advise the driver when the pressure drops below a prescribed level.Differing from conventional devices, the TWD-III features a 7-step digital display (at a pitch of 0.1 kgf/cm2) which shows the pressure of each tire within an optional range, and it also has a flat tire warning function.The employment of echo effect from clystal vibrator resonance precludes the need to attach a power source on the tire. The pressure sensor is installed in place of wheel center cap designed so that a double headed magnet driven by tire pressure changes turns reed switches on and off. The resulting combination of three clystal vibrators determines the pressure indicator in 7-step. It is further designed to compensate centrifugal force in the wheel so that no fluctuation in the reading is caused by changes in speed.Compared with conventional ones, this new device provides high prospects for enhancing driver convenience while maintaining a high level of preventive safety and vehicle performance.