Most are mini transmitter devices. My wife's car still has the blue
stickers on the rims with the transmitting frequency printed on them. (BMW
750)

However, not all are. Are you ready for this? Here's how it's done on my
BMW M5:

You, or the dealer inflates the tires to the proper pressure.

You enable the flat tire monitor (on BMW's this is done through a computer
interface called "I-Drive". This calibrates the system.

As you drive around, a sensor on each wheel records the wheel rotation and
compares it to the other three wheels. Each sensor compares to the other
sensors, in other words. If one tire starts going flat, it's number of
rotations per unit distance will be different than the other three wheels.
If this difference goes out of a predetermined window (allows for turns,
etc.) the flat tire alarm will notify the driver.

I had to read this three times in the manual before I'd believe it.
German engineers have a reputation for getting carried away, but this is
ridiculous.


That seems really odd. On my Infiniti, there is no calibration to proper
pressure and then a warning if you fall outside the accepted range of the
calibrated pressure. The Infiniti's system seems a bit more
straightforward. By switching to one of the ancillary screens on the LCD
nav display, I simply get a real-time digital readout of actual tire
pressure in psi of all four wheels on the ground. As the tires warm up, I
can see the pressure increase correspondingly. The left front may read 34
psi while the right rear may read 33 psi, etc. If any tires falls below an
absolute pressure 29 psi, a warning screen will automatically pop up on the
nav display. Seems a lot more simple and intuitive than any other system
I've seen described in this thread.