The best way to conserve fuel is to play in the Asiatic fleet!
2/3rds is indeed "fuel conservation" cruising mode and will get you anywhere in theater from Midway with enough to patrol and easily get back... the stated range of most WWII fleet boats is about 11k nm at 10 knots (or 2/3rds) which I think is pretty accurately depicted in SHIV. Is it the fuel consumption at other speeds that's out of whack?