Sublight factor was the primary means of measuring speeds sublight attained using impulse drive. Alternatively, warp factors, expressed as lower fractional values, were used to measure sublight speeds. (Star Trek: The Motion Picture)

As the USS Enterprise traveled from Elas to Troyius in 2268, Captain Kirk delayed the journey by traversing the Tellun system at the sublight speed factor of .037. (TOS: "Elaan of Troyius")