The speed of light was, according to Amelia Rumford, 180,000 miles per second, (TV: The Stones of Blood) or, according to Eugene Jones, 299,792,458 metres per second. (TV: Random Shoes)
Despite the theory of relativity stating otherwise, (TV: The Stones of Blood) it was possible to travel faster than this rate; this was called faster-than-light travel. Monarch attempted to travel at this speed and travel backwards in time. After stopping him, the Fifth Doctor stated that such a thing was impossible. (TV: Four to Doomsday) The Third Doctor, however, seemed to think it was and was even experimenting on it himself. (TV: The Claws of Axos)
According to Adric, the Theory of relativity, e=mc2, meant "energy equals mass times the speed of light squared". (TV: Four to Doomsday)
The speed of thought was equivalent. (PROSE: The Sands of Time)
During the Last Great Time War, the Daleks once plotted to destroy Gallifrey by crashing the Thousand Worlds into the planet at fifty times the speed of light. (AUDIO: The Heart of the Battle)