The goal of vibration testing is to obtain a good representation of the mode shapes and associated natural frequencies of a mechanical system. Often the test setups for vehicles are designed to approximate free-free boundary conditions. Ideally the stiffness and mass of the actual mounting system should be attached so that the measured mode shapes and frequencies of the test structure are unchanged. In practice, this is not possible and the quantitative influence of mounting stiffness on measured frequencies is of interest. Two models have been analyzed and will be discussed in this paper. First, a model having one vibrational degree of freedom was chosen because it is the easiest system to analyze and because it represents an individual uncoupled mode of a multiple degree-of-freedom system. Second, a whole car was analyzed to provide a realistic assessment of the influence of mounting stiffness on a vehicle vibration test.