Stay afloat as much as possible and go 2/3 speed or less if you really got time to kill. Plotting your route affects fuel consumption. The shorter route obviously uses less fuel. Submerging and resubmerging uses fuel to recharge. I only submerge during air attacks and getting away from those dreaded DDs. Try using RFB or TM mods with the ones you have. It could make the difference.
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