Run on the surface at 9 to 9.5 knots(the most efficient speed for the Gar is around there). It's much more efficient to run purely on the surface vs. running submerged and only recharging batteries on the surface. The above two tips should get you to East China Sea with about 75% of fuel remaining. Topping off at Midway is also something you should do both on the way in and out if you want the maximum range
