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This study investigates how terrain deformability affects the performance of heavy forestry vehicles using simulation-based analysis. A Komatsu 895 forwarder was simulated in AGX Dynamics across three test scenarios including obstacle traversal, turning, and uneven terrain. Each scenario was evaluated on both rigid and deformable terrains with varying soil deformability, vehicle speed, and load. Results show that terrain deformability is the primary factor influencing vehicle behavior. Higher compression indices lead to increased fuel consumption, deeper rut formation, and more unstable dynamic wheel-ground interactions. For example, rut depth increased from 4 cm to over 50 cm as terrain became softer, and fuel consumption rose by up to 4 times on highly deformable terrain. Vehicle speed had a stronger effect than load, with peak rut depth decreasing by 40% and average compaction by 30% when speed was increased from 1.0 to 2.0 m/s. These effects are consistent with the time relaxation mechanism implemented in the AGX terrain model, where longer wheel-soil contact duration causes greater deformation. The study highlights the strengths of the AGX deformable terrain model in capturing vertical compaction effects but also its limitations, most notably, the absence of lateral soil deformation modeling. Future work could include a comparative analysis using a new AGX terrain model that incorporates shear deformation to more accurately represent complex wheel-soil interactions. Additionally, experimental validation using field measurements from real forwarders could be explored to strengthen the findings.