[jadnashuanh]

RFT don't change their diameter all that much when there is zero pressure, thus my surprise that BMW uses the ABS to detect it. On those vehicles in the US with the in-tire sensor, it triggers with a certain pressure change, way before there's any visual indication, although I'm sure the computer is more observant than I and the rotation rate does change a little. I'm still surprised it can do it on a RFT reliably until it is quite a bit lower than ideal.

One thing I do not like about mine verses those in my last car, is that it only indicates a color - the old one showed the actual pressure, not the stoplight that BMW uses.

FWIW, I was out on our first really cold day, and the car, after driving a little out of the warmer garage, showed all four tires as low...so much for alerting to an incremental drop like in the other car...each tire was about 4 pounds lower than when initialized. Air pressure drops a fairly consistent amount as the pressure drops. Had I been driving faster/longer, they probably would have warmed up enough to turn the alert off. In this instance, the computer couldn't use differential rotation rates between tires, it would have had to remember it when the system was initialized.