Cruising via standard speed is not the most economical way to get the most mileage out of your fuel. The slower you go the farther you can get with the same amount of fuel. Cruising at slow speed your range increases tremendously; the downside being it takes a lot longer in real time waiting to get from point A to B (even at 1024x speed).
If you want to squeeze more mileage using the batteries you just need to cruise submerged till they hit 10% or less and then surface to recharge. Once they hit 100% submerge and repeat. This works well if you burn up too much fuel on patrol and need a few more hundred k's to make it back to dock; its a lot of work to do.
