This study evaluates and optimizes the tooth-root bending fatigue strength and root temperature of plastic spur gears proposes a concise design workflow that couples a Taguchi L18 mixed‑level design with grey relational analysis (GRA) and VDI/ISO‑based calculations verified by finite‑element analysis (FEA). The studied factors include module (m), face‑width ratio (b/m), profile shift (x), root fillet radius (ρ/m), normal pressure angle (α), and friction coefficient (μ). Finite Element Analysis (FEA) was employed to determine stress and temperature distributions under realistic loading conditions. An L18 orthogonal array was used for experimental design, and the Grey–Taguchi method enabled multi-objective optimization of conflicting responses. The results show that the module and face-width ratio exert the most significant effects on tooth-root bending stress, whereas the fillet radius strongly influences temperature rise. The proposed Taguchi–GRA approach demonstrates high accuracy with negligible lack of fit, offering a practical and reliable strategy for the design of lightweight and durable plastic gears in machine engineering applications.