This study investigates the impact of material composition and processing parameters on glass fibre-reinforced recycled polypropylene (rPP) flakes through mechanical and rheological testing. Using a three-factor, three-level Box-Behnken Design (BBD) within Response Surface Methodology (RSM), the effects of glass fibre flake content, additive content, and extruder screw speed on the properties of injection-moulded samples were evaluated. Analysis of variance (ANOVA) and cross-validation were conducted to assess the influence of these parameters on tensile and rheological properties. The key objective was to maximize tensile modulus (E _t) without compromising tensile strength. The results showed a 60 % increase in E _t by optimizing extruder screw speed and flake content, achieving the highest E _t (4.1 GPa) at a screw speed of 150 rpm, 50 wt% flake content, and 5 wt% additive. The additive contributed up to a 20 % increase in E _t but primarily influenced rheological behaviour. Glass fibre flake content was the most significant factor affecting mechanical and rheological properties, with additive content enhancing flow characteristics. These findings highlight the potential to optimize glass fibre-reinforced rPP composites for improved performance, offering a sustainable material solution for applications in diverse industries.