Warp drive – Wikipedia

Warp 9 in The Original Series was the “never exceed” speed for the hulls and engines of Constitution-class starships, equivalent to the aircraft V NE V-speed. Warp 6 was the V NO “Normal Operation” maximum safe cruising speed for that vessel class. Only five stories in the original Star Trek series involved the Enterprise traveling beyond warp …

https://en.wikipedia.org/wiki/Warp_drive