Yea there's some issues with speed perhaps because campaign and combat have totally different rules and are different games that don't yet work as one together.
-Campaign speed is just a few integers between 6 to 11.
-Emergency burn speeds which adjust these integers by certain factors
-But ship speeds in combat do not relate to their campaign map speeds. In Campaign a fleet of Cerberi (burn 10) can keep catching a fleet of Kites (burn 9) but in combat, the Kites (speed 140) can keep outrunning the Cerberi (speed 110)
-Then there's whatever means the dialog box uses to determine whether you are fast enough to pursue the enemy fleet. Is it burn speed? Is it combat speed of your ships?
I agree with you, if you're fast enough to catch the fleet, then you're fast enough to catch the fleet!
The system of dialog boxes and engagement options are abstractions that don't make sense and ways to avoid making the drastic alterations required for the game to actually make sense. There would have to be sweeping changes to the way campaign speed relates to combat and ship system speeds.
