<p>Estimating the maximum speed capability (athletic capability) of very large fossil animals is challenging. While large animals have limbs favoring longer stride lengths and higher speeds, their body mass imposes limitations on bones, joints, available forces, and physiology, resulting in the fastest animals not being the largest. Here we use the well-known relationship between body mass and potential maximum speed to calculate upper limits for the athletic capability of fossil giant proboscideans and sauropods. First, we assess the reliability of two different fits to maximum speed data for mammals. Subsequently, we analyze masses and speeds of live proboscideans, finding they consistently move below the lower bounds predicted by model fits. Finally, we propose maximum speed for representative fossil proboscideans and sauropods. Although calculated maximum speeds for larger proboscideans are comparatively lower, there is substantial overlap with those observed in live proboscideans. For the largest sauropods approaching or exceeding 50 Tn our calculated maximum speeds are around 10&#xa0;km/h or lower. These findings suggest that immense body size and graviportal structure of sauropods were key factors likely restricting their locomotion to a single, steady gait, consistent with fossil trackway evidence.</p>