I want to buy new rims and tires for my 2005.5 A 4 3.2. How does the tire pressure monitoring system wokk? Is it in the valve stem or inside the tire? How can I replace this to make this work for my new rims without messing up my factory rims because I want to use them in the winter months?? Also I have 17 x 7.5 Rims. Can I use an 18 x 7.5 or 18 x 8 with out messing up my speedometer???