c (light-speed)

c, or the speed of light, is the speed of which photons travel in a vacuum, A.K.A. the fastest speed in the universe.
To travel at/exceed this speed would result in breaking the fabric of space-time.
c = 299,792,458 meters per second = 671,000,000 miles per hour

The new spaceship, Exr-C2 can travel at 0.6 * c (light-speed), or 60% of the speed of light.