Welcome to Subsim! Armistead has you up to speed on how to install a mod there.
But that won't solve your fuel problem. First, have confidence that you have plenty of fuel to go where you need to. You just need some fuel management skills there.
Your sub has wildly varying ranges depending on your throttle settings. It sounds like you are charging around at ahead full or ahead emergency. That will drain your tank just as well as a 5" shell in the fuel tank!
Perhaps you are charging around wide open and don't even realize it. How could such a thing happen? People playing the ostrich strategy love to hide all day submerged at slow speed and travel on the surface only at night. But I'll bet you didn't know that for all those battery charging hours your engines are running wide open to charge batteries, while your boat travels whatever speed you set it! There's that 5" hole in the fuel tanks again. Your submarine is a surface craft with the ability to submerge in an emergency or for stealthy attack. Not being attacked or not attacking? STAY ON THE SURFACE!!!
Your most economical speed is 9 knots. At any speed above or below that you will travel fewer miles per unit fuel burned. At 9 knots, ahead standard in most mods, you can easily return from Japan to Pearl on 30% fuel remaining. You can make Midway on a lot less than that.
Fuel management is initially frustrating, but it becomes so automatic that later you won't even think about it. You'll see posts like yours, scratch your head and say, "How could that possibly be a problem?"
Hope you stick with the game and get to that point soon!
