Which insect is the fastest runner?

The Australian tiger beetle.
Australian tiger beetles, genus Cicindela, subgenus Rivacindela, (Coleoptera: Cicindelidae) are the fastest running insects known. The fastest, Cicindela hudsoni, can run 2.5 meters per second (5.6 miles per hour).

It has been commonly thought to be the American cockroach, Periplaneta americana- but they only reach 1.5 m/s (3.4 mph).