From Pearl to Honshu in a gato class I travel at 1/3 speed ( 8 Kts in fair weather ) during the night hours and 1 KT at P depth during the day hours with plenty of TC.
Have always had no less than 1/4 tank upon entering Midway on return to Pearl.
Run very few mods:
Rest is stock 1.4. Also using your fuel this way you will have a reserve to run "Ahead Flank" when on surface when you might like to overtake a Maru or run from a DD ( well you can try anyway
) and still have plenty fuel to find a refit base.
Unless you somehow got your hull looking like rat holes
then you better find a very slow speed and pray.
