You are misunderstanding what mission speed means. Mission speed is not actually a speed that the ships travel. You need to know first what the numbers in the parentheses mean.
The 8 is the maximum speed they can move.
The 4 is their cruise speed, which is their guaranteed movement per phase ("guaranteed", with some caveats, but if nothing happens they will move this speed).
Note that if you change the TF to Full Speed, those numbers will change to (8/8). If you change the TF to Cruise Speed, they will change to (4/4).
If you wanted them to move 12 hexes, you needed to set them to Full Speed, yes. Mission speed means they only might move at a speed faster than cruise if the TF commander believes the situation calls for it.
You should re-read the manual on what the speed settings mean.
Instead of reading the manual, which isn't always clear, I can have a good gentleman such as yourself respond to a question posted on-line, as you did.
Btw, I will challenge you to a "grudge match", team, and you had better accept. I will post your thrashing in the AAR forum.
you my Huckleberry?