The mystery of just how fast the king of the dinosaurs could run might have been solved by a science team from the north of England.
The Manchester University team worked out that Tyrannosaurus Rex probably had a top speed of around 18mph - making it faster than many sports stars.
T Rex might have been the fiercest dino, but it wasn't the fastest.
Compsognathus pelted along at 40mph - 5mph speedier that the current fastest creature on two legs, the ostrich.
Compsognathus - Weight: 3kg / Speed: 39.8mph
Velociraptor - Weight: 20kg / Speed: 24.2mph
Dilophosaurus - Weight: 430kg / Speed: 23.5mph
Allosaurus - Weight: 1.4 tonnes / Speed: 21mph
Tyrannosaurus Rex - Weight: 6 tonnes / Speed: 17.9mph
The scientists used a supercomputer to calculate their results. They put in details about weight, bones, muscles and other stuff and the computer worked out how the dinos were best able to move.
The computer started with the first clumsy steps of the dinos and eventually worked out the top speeds of the creatures when they were at their fittest.
T Rex's speed has fascinated scientists for many years, with some believing that the creature's huge leg muscles would have prevented it from running too fast.