The standard measure of (car) fuel efficiency in Continental Europe is litres per 100km. So a relatively thisty car (like mine
) might do 10, a diesel with decent performance (say a BMW 320d) would be touted at about 6.5, a Prius I believe is supposed to be down at 3.5 and they are talking about the next generation of super-efficient cars getting down to 3.
Obviously, with just one occupant (ie load factor of 20%) these figures are equal to litres per 100 passenger-km.
Without looking up the figures, I believe that a representative figure for a long-haul flight in a modern aircraft with a typical l/f would be about 4 or 4.5 which would be as good or better than a VW Golf on a long journey. Next generation aircraft will hopefully get that down to 3 (better than a Prius!)