A study suggests that at six tons and 40 feet in length, the two-legged T. rex was so big its leg muscles could not have let it sprint.
Using a computer model to calculate how much muscle an animal needs to move at different speeds and postures, two California researchers estimated that T. rex was capable of moving in the range of 11 mph to 25 mph, with the lower figure more likely.
That is far less than the 45 mph some scientists have suggested.
"Twenty-five mph cannot be ruled out, but it's really straining credulity. It would require such enormous power it would be pushing its limits or going beyond its limits," said John R. Hutchinson of Stanford University.
The conclusion may have a bearing on the long-running debate over whether T. rex was a predator that chased down its prey or a scavenger that fed on carcasses.
Hutchinson collaborated with Mariano Garcia of the University of California at Berkeley. Their findings were published in Thursday's issue of the journal Nature.
Dinosaur experts were split on the team's conclusions.
Hutchinson and Garcia tested their computer model on alligators and chickens — two relatives of dinosaurs. It correctly predicted that alligators' leg muscles were too small for them to run quickly and that chickens' leg muscles are more than adequate for fast running.
When the same model was applied to T. rex, the researchers found that to move as fast as 45 mph, its leg muscles would have had to make up 86 percent of its weight. That would have been impossible, because it would have left only 14 percent for the rest of T. rex's enormous body.
University of Maryland paleontologist Thomas Holtz Jr. agreed with the team's findings but said the approach should be tested on larger animals such as ostriches that are closer in size to T. rex.
Chris Brochu, a paleontologist at the University of Iowa, praised the paper as "the firmest statement yet made on how fast a large bipedal dinosaur could have moved — its running ability or lack thereof."
But he said the study does not mean T. rex was too slow to prey on other large beasts such as triceratops. He said the limitations that may have kept T. rex from running fast apply to other big dinosaurs, too.
Other scientists were unconvinced by the study.
Martin Lockley, a professor of geology at the University of Colorado at Denver, said the computer model cannot reproduce the complexities of the hulking T. rex. "Animals are not machines. These animals were dynamic and flexible, and tissue is very hard to reduce to numbers," he said.
Julia Day, a paleontologist at Cambridge University in England, said that among other things, T. rex may have weighed a lot less than scientists had thought.
"They're not taking everything into account," she said. "There are just so many unknowns."