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Playing stock you need to run everywhere at 2/3 speed. That will give you best fuel efficiency. When using batteries always run at slow speed, unless a destroyer is passing overhead. He's about to drop depth charges, and you need that burst to get out of the way. Other than that, go slow. Flank speed takes at least four times as much fuel as standard speed, and up to eight times as much as slow speed. I'll reiterate - at slow speed your batteries should last 48 hours; at flank speed they'll last about two hours.