With the requirement put in place in 2007 for every vehicle to be equipped with a tire pressure monitoring system, the standard method involved having a transmitter on each wheel. This transmitter emits a wireless signal that the vehicle computer receives and processes. At a minimum, this transmission contains the tire pressure and an ID code. This ID code helps your car determine which signals belong to its tires since there are likely more than just four bouncing around at any given time. These transmitters tended to be quite expensive, sometime over $100 per wheel. The cost of sensors in addition to a sometimes complicated reprogram procedure could make purchasing a Tire & Wheel Package for winter cost prohibitive.
Prior to the mandated systems, there was another method deployed in some vehicles called an indirect or ABS based system. These systems monitor the wheel spin rate through a speed sensor typically part of the ABS system. When a variance is detected, for example one wheel starts spinning faster than the others, it triggers the light. This is because that one spinning faster must be shorter than it was, that is to say it lost some air pressure. The beauty of this type of system is the lack of a transmitter on the wheel itself, removing the cost burden when considering a winter package.
These friendlier indirect systems were abandoned with the advent of the TPMS mandate. But in 2013 and going forward, VW reintroduced them on many of their vehicles. This is great news for the consumer. If you own a newer Volkswagen and previously decided against a winter package, it may be time to re-think that decision. For more info on winter packages be sure to read "Why Buy Dedicated Wheels and Tires for Winter."