Phase 1 of the NHTSA Tire Aging Test Development Project consisted of the analysis of six different tire models collected from privately owned vehicles in the Phoenix, Arizona USA metropolitan area during the spring of 2003. This study was conducted to provide a better understanding of service-related tire degradation and to serve as the “real-world” baseline for the development of laboratory-based accelerated service life test for tires (often referred to as a “tire aging test”). On-road tires and full-size spare tires, as well as the corresponding vehicle information were collected through 22 Phoenix-area retailers in exchange for new tires at no charge to the study’s participants. Within the six different tire models studied, a total of 174 on-road tires and 9 full-size spare tires retrieved from Phoenix of varied ages and mileages were compared to 82 new, un-used versions of themselves. The tires were either subjected to one of two whole-tire roadwheel tests or cut apart for material properties analysis. The results were correlated against the absolute age and mileage (if original equipment) of the tires and will be discussed in this paper. The raw dataset and project notes are available for download at (VIN redacted for vehicle owner privacy): http://www-nrd.nhtsa.dot.gov/vrtc/ca/tires.htm