Lightweight design is essential in modern automotive systems to improve energy efficiency, reduce emissions, and enhance performance. This study presents a simulation-driven framework for the lightweight design of an automotive reducer housing using finite element analysis (FEA) and topology optimization (TO). A baseline reducer housing is analyzed under multiple load conditions, including maximum torque, emergency braking, and cornering. Stress distribution and deformation behavior are evaluated to identify structurally redundant regions. A Solid Isotropic Material with Penalization (SIMP)-based topology optimization method is applied with a volume reduction constraint to minimize compliance while maintaining stiffness. The optimized topology is reconstructed into a manufacturable design considering casting constraints. Comparative FEA validation shows significant mass reduction while preserving structural integrity, safety factor, and stiffness. The proposed methodology provides an effective and practical framework for lightweight automotive component design.