The automotive sector’s transition toward sustainable mobility necessitates the adoption of lightweight, eco-friendly materials that reduce energy consumption without compromising structural integrity or manufacturability. This study investigates the performance and energy efficiency implications of flax fibre reinforced polymer (FFRP) composites in automotive interior applications. FFRP specimens were fabricated using vacuum-assisted compression moulding and evaluated for density, tensile and flexural strength, moisture absorption, and thermal stability. Results indicate that FFRP composites exhibit an 18–24% reduction in density compared to conventional glass fibre/polypropylene and ABS interior panels, while maintaining comparable stiffness and impact resistance. Life-cycle weight-saving projections demonstrate a 6.5–8.2% improvement in fuel efficiency and a proportional reduction in battery energy drain for electric vehicles. The findings validate FFRP as a technically viable, environmentally sustainable alternative for non-structural interior components, with surface modification and hybrid reinforcement recommended for enhanced moisture and thermal resistance.