You carry your speed from the turn before. When you move on your turn, leave behind a …ghost‚ marker on the square you left. At the beginning of your next turn, put a marker ahead the same distance as between your current location and the ghost marker. You can move your ship anywhere within one movement distance from there as a 4SA (a single move) or double the distance (a double move). This allows you to attain incredible speed, but you should start decelerating ahead of time.
With Physics on, you do subtract your mobility penalties from your speed in squares, as in the atmosphere. This represents the slower acceleration of clumsier vehicles.
Missiles move at a speed of 600m per round (they are always Double Moving).