Set of two ALUMINUM transmitters for towed cars. ALUMINUM TRANSMITTERS ARE NOT FOR USE ON RVs, FIFTH WHEELS or CARS/TRUCKS WITH BRASS OR STAINLESS STEEL VALVE STEMS! FOR YOUR TOWED VEHICLE ONLY: If your towed vehicle (car, SUV or light truck) was manufactured after 2007, it may have a built in TPMS and aluminum valve stems. If so, you will need aluminum transmitters for your towed vehicle. PLEASE CHECK TO SEE IF YOU HAVE BRASS OR ALUMINUM VALVE STEMS.