The study reported here was conducted to determine the in-use condition of automobiles on United States highways as a function of age, mileage, model year, make, and model. In order to minimize the variances that ensue when data are collected from various sources, criteria were developed for selecting qualified diagnostic centers. Standard component test criteria were also developed for evaluating tested components as either “pass” or “defective.” A vehicle-condition computer data model was constructed to verify vehicle outages at the subsytem, component group, and component levels. The computer program performed a preliminary statistical analysis of the data parametrically and also generated plots of average percentage outages against specified parameters. The validity and reliability of the data assure that statistical findings are at least significant at the 5% level. The results show that vehicles in states with periodic inspection are in much better condition than those in states with random inspection.