Animals minimize cost of transport by selecting gaits tuned to the natural frequencies of their body parts. This allows estimation of tail natural frequency and preferred walking speed (PWS) of T. rex, using an approach we introduce as the Natural Frequency Method. During walking, the tail was subject to flexion torque due to gravity and contractions of the tail musculature, which was counteracted by the caudal interspinous ligaments. Tail heaving with each step caused peaks in ligament strain, making them an important site for elastic energy storage. This is comparable to the nuchal ligament’s role in ungulate neck movement during walking. Based on high-fidelity 3D scans of the caudal vertebrae of adult T. rex specimen RGM.792000, we constructed a biomechanical model of the ligament-suspended tail and determined its natural frequency (0.66 s^-1, 0.56-0.79), which we combined with step lengths from trackway data to find PWS (1.28 m s^-1, 1.09-1.54) for T. rex. Various methods to reconstruct locomotor abilities of extinct dinosaurs have led to conflicting results. Due to the uncertainties involved, it is important to explore independent lines of evidence. Our method explores dinosaur PWS without being affected by estimated muscle mass or hip-height, and therefore opens up a new research avenue within paleo-biomechanics. Our results are closely in line with extant taxa, regardless of size and locomotor mode, which may suggest that previous methods that neglect tail dynamics are overestimates. Steady-state locomotion has a large effect on an animal’s ecological niche, and our method can therefore provide us with new insights into possible gait patterns, habits and locomotor ability of dinosaurs.