I had the same problem with SHIV, in SHIII that was the easiest and most accurate way to gauge speed, and I was pretty miffed when the recognition manual didn't have that information. Anyway, within the game box is both a manual that has ship lengths and a foldout poster that has lengths. If for whatever reason you don't have either of those two, if you do a search on this forum for "calculate speed" or something similar, you'll find a post that has length stats as well, and they are already multiplied by a conversion factor that will give you knots when you divide by seconds.
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