The optimal speed for the Gato, Porpoise, Salmon and Balao class is 9knts, for the S Class is 7knts.
Otherwise your endurance is quite halfed...
Furthermore, if you manually edit your savegames you can have a new fuel load in this way:
1) In your document folder enter the following path: SH4\data\cfg\SaveGames
2) search the save game directory of your last save, lets say 00000002 and enter inside...
3) edit the file GameplaySettings.cfg and change:
LimitedFuel=true
in:
LimitedFuel=false
4) save and return in the directory 00000002
5) enter in the directory UPCInitial
6) edit the file GameplaySettings.cfg also here as per point 3 and save
7) now you can load the save from the game and you have unlimited fuel. If you want you can save the game, exit and re-modify the savegames in order to have the difficult settings as they were initially, but the fuel in your tank remains!!!!!
Enjoy,
Mith