Conventional tire pressure monitoring systems (TPMS) measure the air pressure in each tire of a car with a sensor mounted on the rim. In future, advanced tire monitoring systems (ATMS) will measure additional data like tire temperature, contact area, vertical load, and slip angle. As a consequence, ATMS modules must be mounted directly at the tire tread. Furthermore, tire identification and lifecycle management via radio frequency identification (RFID) will be enabled. To design appropriate antennas for these applications, it is mandatory to investigate the interaction of antennas with the tire. Therefore we explore the tire structure and the dielectric material properties of the tire rubber. In this contribution the characterization of the dielectric tire materials using an open-ended coaxial probe is presented. We measure the reflection coefficients for each rubber tire layer and determine the relative permittivity and loss tangent by a numerical curve fitting method. The dielectric properties of each rubber layer are measured for a standard and a run-flat tire.