The ferocious Tyrannosaurus Rex was too big to sprint and couldn't outrun most humans on a pushbike, research has found.
Scientists have worked out the dinosaur's maximum speed for the first time and found it was only 16.5 miles per hour (26.5 kph) - similar to the average human's.
Large animals take longer to accelerate, so smaller ones like the cheetah set the records, the researchers said.
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Scientists have worked out the T-Rex's maximum speed for the first time and found it was only 16.5 miles per hour (26.5 kph) - similar to the average human's. Pictured is the T-Rex in the 1993 film Jurassic Park
Larger animals run out of energy supplied by the muscles before being able to reach their theoretically possible maximum speed.
This is because the acceleration phase requires muscles to function anaerobically, without oxygen, during which only limited stores of energy are available.
Small to medium sized animals accelerate quickly and have enough time to reach their theoretical maximum speed.
But large animals are limited in acceleration time and run out of readily mobilisable energy before being able to reach their theoretically possible maximum.
A study of almost 500 species from experts at the German Centre for Integrative Biodiversity Research in Leipzig, Germany, found animals only have so much time to reach maximum speed.
This is because acceleration comes from working in short bursts anaerobically - without oxygen.
And as this time is limited big beasts will never match medium sized ones.
The latest formula provide the best theory yet for why bigger animals are slower, a phenomenon scientists have previously struggled to explain.
The same pattern also occurs in air or water with marlins and peregrine falcons the quickest fish and birds, respectively.
The speed of dinosaurs has been debated for decades. Some scientists say T-Rex could reach 45 miles per hour (72 kph) while others have suggested it was more like a plodding cyclist at 11 miles per hour (17.5 kph).
Now the study, published in Nature Ecology & Evolution, shows T Rex would never have been able to keep up with a Jeep as it did in Jurassic Park.
Zoologist Dr Myriam Hirt, of the German Centre for Integrative Biodiversity Research, Leipzig, said: 'Palaeontologists have long debated the potential running speeds of large birds and dinosaurs, that roamed past ecosystems.'
Her unique formula combined body mass, power and time needed for acceleration.
This graph shows the relationship between Body Mass and Maximum Speed of Dinosaurs using a new mathematical model developed by scientists. The results found that the medium-sized raptors were fastest and that the T-Rex was slower than first thought
It calculated T Rex's speed at about 16.5 miles per hour (26.5 kph) - more than twice as slow as the much smaller Velicoraptor which could have got up to 34 miles per hour (55 kph).
The average sprinting speed of a human is about 15 miles per hour (24 kph).
Dr Hirt said: 'This is consistent with theories claiming Tyrannosaurus was very likely to have been a slow runner.'
The essence of her hypothesis is simple - animals have only a finite amount of time to accelerate from a standing start