How fast was T.rex?

One American dinosaur expert claims that T. rex was able to 'cruise' at 60 kpm (40 mph) on the basis of the structure of its legs, which in turn implies an even higher top speed sprint. Most palaeontologists find this a little extreme. For very short bursts of speed - such as leaping out to ambush an unsuspecting victim - it may have been able to reach speeds of 20-30 mph, but this would have been a very brief burst of speed. It seems unlikely that there huge dinosaurs did anything but amble around at slow speeds (averaging about 5/8 mph) for much of the time.