Why Tyrannosaurus was a slow runner and why the largest are not always the fastest

No other animal on land is faster than a cheetah – the elephant is indeed larger, but slower. For small to medium-sized animals, larger also means faster, but for really large animals, when it comes to speed, everything goes downhill again. For the first time, it is now possible to describe how this parabola-like relationship between body size and speed comes about.
Paleontology News — ScienceDaily