When fuel is used the fuel display shows the decrease. If you save and reload, the fuel appears to go to maximum again. This is in fact inaccurate. I was in the position of having very little fuel, I saved and reloaded to find I had full fuel. On embarkation the rate at which fuel was used for even the most minor manoeuvre was huge. I believe that this shows that the display is not showing the fuel level relative to the max fuel tank size, but only relative to the amount of fuel when the game was started.
Vegastrike V0.5.0 (Linux)
Anonymous
It's enough to change the ship (in "My fleet") and then change it back.
Then it undocks with "full" fuel indicator... dropping rapidly if the fuel was low.