Engineering specifications, i.e. test bogeys, are criterion for determining the success or failure of durability designs in the product development process. Considerations in the development of the specifications for vehicle structural components, such as axle housings and suspension torque rods, have been presented in a previous SAE paper . This paper has been prepared because the factors on the same subject for vehicle drive train components, such as gears and bearings, are quite different. The center of this study is on “how to define equivalent duty cycles for lab test”. Several issues distinguish this task for drive train components: High cycle fatigue, high accelerated tests, competitive failures and failure modes, empirical component load-life data, loading, field correlation, and system level tests. To further complicate the issue, drive train components are often selected from available internal and external sources, and the system level design validation is accomplished in steps. Ideally, what we need is a flexible and reliable approach that can be used routinely and cost effectively for design validation. One method outlined in this paper is to conduct the integrated simulation and modeling in which vehicle and power train performance simulation and fatigue life estimation are conducted along with test correlation. To illustrate, a scenario study of a test development exercise for a commercial vehicle's final drive system is provided.