Hmm, this is a good opportunity to explain all this WC speed craziness -- as I'm sure I've made very little sense thus far.
The long version:
First, a very quick overview of exactly how Wing Commander ships are propelled in the first place. Four important components are responsible for making a ship 'go':
1. Powerplant: Ships in the Wing Commander universe are built around fusion powerplants (exception: older Kilrathi fighters use a highly expendable nuclear powerplant). These fusion powerplants consist of an electromagnetic field. The field is generated by regular electromagnets and then amplified and directed by magnetic monopoles. Hydrogen is released into the field and then supercompressed -- creating "hot fusion". The energy given off by the powerplant is then used to power ships systems, including the drive unit.
2. Engines: There are two main types of engines used on Wing Commander ships. Most fighters mount ion drives, which ionize supercompressed hydrogen to generate thrust. Capital ships (and a few notable exceptions) mount far more complex Matter/Antimatter drives, which react the supercompressed hydrogen put out by the fusion drive with anti-matter to generate thrust.
3. Ramscoop: A second electromagnetic field (the "primary proplusion node") plays an important role in propelling a Wing Commander craft -- all ships generate an enormous (several square kilometers in size) electromagnetic ramscoop (note: fighters generally have only one field, which doubles as both a ramscoop and fusion plant). The field sweeps the area ahead of and surrounding a ship clean of spaceborn hydrogen. The faster a ship goes, the more hydrogen is taken in -- and the more drag is created.
4. Containment Vessels: Hydrogen is funneled through a ship's intakes ("flux resonance tubes") and stored in an internal fuel tank. A series of plasma coils keeps the hydrogen in stasis until it must be passed through a fuel control collar into the fusion powerplant. A ship uses its onboard fuel supplies *only* to accelerate, decelerate and to compensate for drag created by the ramscoop. This supply is refreshed by hydrogen atoms continually from the ramscoop. A ships *maximum* speed, as listed in Joan's Fighting Ships, is that at which (during optimal conditions) an exact parity between fuel use, hydrogen intake and drag is reached. For a massive capital ship the average is about 150 kps -- for a fighter, around 500 kps.
Several factors limit the actual speed of a ship. Most importantly, competition for hydrogen. During combat (or, to an even greater extent, around large capital ships) hydrogen is limited. Instead of a single ship gathering all the hydrogen in several square kilometers you have several (or hundreds or thousands). Speed is limited (according to the targetting VDUs in Wing 1 and 2) to about 1 meter per second rather than 1 kilometer per second (thus a Ferret would travel at 500 meters per second, a Devestator at 320, and so forth and so on).
(Side note: during atmospheric flight, internal fuel supplies alone power a fighter. Fighters in an atmosphere generally maintain a speed of 500 meters per second.)
(Second side note: afterburners don't function the way you think they do. They do not draw on onboard fuel. In activating the ship's afterburners, two things occur: first, the size of the ramscoop's opening is reduced -- lessening both drag and hydrogen intake. Second, the hydrogen which is taken is is syphoned directly to the engines rather than the fuel tanks. Total thrust is doubled by exactly 50% -- with the previous level being maintained by the onboard fuel tanks. Thus the onboard fuel is depleted only to maintain the ship's previous thrust.)
(Third side note: large capital ships frequently operate with 'scoops closed', using only onboard fuel to generate thrust. A ship travelling with scoops closed cannot easily maneuver, but can reach speeds of around 10,000 kilometers per second in half an hour. Such operations must be planned, as the ship will also require another half hour and an equal amount of fuel to deccelerate).
The short version:
* During combat, ships travel at around 1 mps per 1 kps.
* Capital ships can generally escape really, really quickly if need be.
* Ships do not have an easily expended source of fuel.