There are conditions where the zombies can run the same speed such as on escape maps where there is a speed cap, but a slower zombie will never travel faster than a faster one unless it is hacking.

For example on some escape maps the fast zombie and predator are the same speed as a result of the max speed limitation. This however will not make any other zombies faster it will just make all of them slower than x amount. On any given map no zombie can travel faster than the fast zombie and no zombie should be going faster than a zombie that is normally faster than it. (only the same speed at most)