I've been seeing alot of studies lately saying that t-rex was probably relatively slow [link|http://news.nationalgeographic.com/news/2007/06/070607-trex-dinosaur.html|http://news.national...rex-dinosaur.html] (hint: if you're being chased, make a lot of turns :)

Of course none of these scientists have seen one run, so it's all subject to debate.