How fast would T. rex run?
from Audrey, age 16, Tal., FL, USA; July 18, 2001

TOM: A VERY interesting question, but one for which nobody has a good scientific answer. Tyrannosaurs have legs which show specializations for running (in fact, tyrannosaur legs are built almost identically to the legs of ostrich dinosaurs (ornithomimids), a group most recognized as among the fastest dinosaurs). So T. rex was probably one of the fastest animals of its body size: faster than other big meat-eaters and faster than plant eating dinosaurs. However, as an animal gets bigger it can suffer much more damage from trips and falls, and so bigger animals tend to be a bit more cautious. Therefor, fully adult T. rex might not have normally run as fast as younger specimens. However, we do not have any good way of measuring HOW fast these speeds would be in terms of miles per hour or meters per second or so forth.

Incidentally, we honestly don't know the speeds of very many modern species of animals. Humans, dogs, horses, and cheetahs have all been run under controlled circumstances on tracks, so we have a good idea of their speeds; many other animals have been run on laboratory treadmills, but these probably do not reflect conditions they would experience in the wild. Most of the speed measurements listed in animal books or in documentaries are actually little better than guesses.