Designing a durability test for an automatic transmission that appropriately reflects customer usage during the lifetime of the vehicle is a formidable task; while the transmission and its components must survive severe usage, overdesigning components leads to unnecessary weight, increased fuel consumption and increased emissions. Damage to transmission components is a function of many parameters including customer driving habits and vehicle and transmission characteristics such as weight, powertrain calibration, and gear ratios. Additionally, in some cases durability tests are required to verify only a subset of the total parameter space, for example, verifying only component modifications. Lastly, the ideal durability test is designed to impose the worst case loading conditions for the maximum number of internal components, be as short as practicable to reduce testing time, with minimal variability between tests in order to optimize test equipment and personnel resources. In this work, customer usage correlation data and fatigue damage models are used to analytically determine expected component level damage values. These values are then statistically evaluated to establish customer lifetime design targets. This paper describes a methodology for creating a combined duty cycle from a collection of routes by reverse deriving above design targets such that the combined duty cycle represents the customer lifetime design targets for multiple transmission components. The combined duty cycle data becomes the key input for designing or modifying a customized automatic transmission system durability key life test (KLT) for a given vehicle program. An analytical description of the method is provided, along with an example to demonstrate the concept.