Just as with cars, larger cruise ships tend to have lower fuel efficiency, at least in gross terms. To give the reader an idea of the fuel efficiency of large cruise ships, I'd like to use the world's largest cruise liner as an example. The MS Oasis of the Seas, completed in 2010, has a total length of 361.8 meters, a height of 72 meters above the sea line, and room in its 16 passenger decks for 6,296 passengers. At its cruising speed of 22.6 knots (26 mph), the Oasis of the Seas burns 11361 gallons of fuel each hour. The fuel efficiency, then, is 0.0023 mpg, or 12.08 feet per gallon!

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This figure seems very low, especially in comparison with mpg ratings for cars. However, keep in mind that a cruise ship carries many more people than does a Honda Civic. The Oasis of the seas, when fully occupied, gets 14.40 mpg per passenger, or 19.36 mpg per person including the crew. This is still a low figure, but not quite as absurd as the overall miles per gallon number.
Image: Wikimedia Commons - Oasis of the Seas