A series of tests were conducted to measure the dynamic stiffness transfer functions between the wheel center of a rim-mounted tire and the contact patch. Of particular interest was the interaction between the tire acoustic cavity mode and the modes of the tire/rim system. By varying the concentration of helium gas within the tire, it was possible to sweep the acoustic resonance through a group of rim/tire resonances. These results showed that there is relatively weak interaction between the cavity modes and the tire/rim modes. It was found that the resonance frequency of the cavity shifts downward with increasing tire load, and that only the z-direction dynamic stiffness is affected by load. Changes in inflation pressure were found to have no effect on the cavity resonance frequency, and increases in inflation pressure led to significant changes only in the x-direction dynamic stiffness. A simple analytical model of a coupled structural/acoustic system was found to produce results similar to those observed in the tire testing.