As previously mentioned, mods have a big effect on various aspects of the game, including fuel. In a Porpoise Class sub, in the stock version, you should get best fuel efficiency at about 9 knots. Also, in the stock version, some of the missions are FUBAR, though I don't know for sure if your particular mission is among the known ones. Either way, if you're low on fuel, top off your tanks first and then complete your mission. Oh and BTW, the radio is not your friend. Don't send in any contact reports unless you want HQ to keep giving you objectives to complete.
