i had made a small program to do just this for you some years back.
someone had uploaded it to filefront it seems:
http://files.filefront.com/SH4+Speed.../fileinfo.html
it was for stock SH4, so i'm not sure it will still work.... seeing as the names of the ships are different in the last mod i played; and the lengths of the ships may have changed.
i may write another one that is configurable instead of hard wired so that it can be kept up to date.
basically, you picked the ship from a list and entered the time it took to pass your crosshairs and it gave you its speed.