Originally Posted by tracer bullet
Not sure the %. I am guessing it's just that, a comparison of wherever it is at a given moment compared to the last reset. I doubt it has any idea of an actual pressure, just that there is or isn't a big change.
Not true, you'll notice that if your tires are too low, aka 25psi or lower for example, that the TPMS will not accept the reset and will still tell you that your tires are low. Most oems use TPMS sensors that provide an actual readout. Not sure what BMW uses but I wouldn't be surprised if its sensors are accurate within 1 psi or so.