This study investigates the influence of infill patterns on the stiffness and the dynamic behavior of spur gears, focusing on lightweight internal geometries achievable through 3D printing. Four alternative infill designs-intersecting lines, circular arcs, honeycomb, and grid - were modeled and compared to a conventional solid design. Finite Element Method (FEM) analyses assessed the maximum displacement of the spur gear under torsional load and the eigenfrequencies of the gear-shaft assembly. Results show that intersecting line and circular arc infills (V1 and V2) reduced maximum displacement by approximately 49% compared to the original, indicating a significant increase in stiffness with minimal mass change. All lattice-infill designs led to higher natural frequencies, reflecting improved dynamic performance. These findings are consistent with recent studies showing that optimized lattice structures can improve strength-to-weight ratios while maintaining or enhancing load-carrying capacity in additively manufactured gears and components. The study highlights the potential of custom infill patterns to produce lightweight, high-performance gears suitable for dynamic applications.