The sperms environment contributes greatly to how fast it can travel during ejaculation. Although the length of a sperm’s flagellum is a big factor to consider too, the speed of sperm is not only dependent on its physiology. Due to its miniscule size, the viscosity of its surroundings can greatly affect a sperm’s swimming speed.
Whether within the male organ or when it’s released into the female sex organ, sperm cannot travel without semen.
The average speed of ejaculation is approximately 45 km/hr. But if you're body is doing much faster than that, so is the semen in you. During sexual excitement, some men can make their sperm shoot over 3 feet. Frequency of ejaculation and intensity of sexual stimulation are also contributing factors.
The speed of the ejected semen is also relative to the ejaculatory penis. In fact, through training the Kegel muscles a man can increase its speed
Knowing the speed of semen is especially vital in developing contraceptive technology. Scientists are looking into the possibility of developing a pill that can slow the sperm down to a point where it is unable to sustain life functions. It could become the "after intercourse" pill. Determining the speed of the sperm would establish the amount of the chemical inhibitors that’s necessary for effective intake.
Edited by Glenn Elert -- Written by his students. "Speed of a Sperm Cell”, The Physics Fact Book.
The Teenage Website. " All About Ejaculation", March 2006.
Dr. John Marshall. " The Joy of Sexual Physics ",2003.