For the most economic speed, I always select unlimited fuel and then flank speed to my patrol area.
I get so sick and tired of leaving Lorient for patrols in the Southern Africa area in my Type VII. 8 Kts at TC max of 256 (so I don't miss contacts or get strafed) is a fun-killer on my part. I would rather take a type II through the Kiel canal.
I like the challenge of realism like every one else but I AM playing this game for fun. Putzing along at 8 Kts @256 is not my idea of maximum yucks.
Sure I will never have the pride of 100% realism but on the other hand I will be able to do more than one patrol per evening