BMW uses two types of systems TPMS and FTM (I think). TPMS uses pressure sensors in the wheel and FTM use the wheel speed sensors to compare rotational speed against the other tires (a tire low on air pressure gets smaller and thus rotates faster as compared to the others). My 1998 E38 (system retrofitted by me) and 2004 E60 uses the FTM system. My 2008 E70 uses the TPMS. I believe in the USA, only the TPMS systems is allowed by DOT regulations at this time. Not sure when the change occurred. For sure, the FTM is the more simple solution, but probably not as sensitive to pressure changes.