The Cheetah has long been recognized as the fastest land amimal in the world reaching speeds upwards of 70mph. Pronghorn antelopes, another animal on the savanna, can reach speeds of up to 60 mph. This animal can sustain these speeds much longer than a cheetah.
To determine the fastest animal in the world one has to consider the nature of the race that you are going to ask them run. Once we get speed on land, however, the most important consideration is going to be the distance over which the race is run.