A 747-400 burns from 1.8 to 3.4 tonnes/hour/engine at cruise, depending on weight. Call it 3400 gal/hour on average. At 500 knots, that's 585 mph. 585/3400=0.17 mpg (5.8 gal/mi). If there are 350 passengers on board, that's 60 passenger miles per gallon -- about the same as the claim for my wife's new Toyota Prius when she's driving to work solo (though so far she only gets 46-48...)!