Puritan,
You went wrong with the maths.
Consider a B737 at 0.68 mpg with 148 passengers aboard.
If you want a "miles per gallon per passenger" figure you need to divide (not multiply!) this by 148 which gives 0.00459 mpg/passenger.
If you want "passenger miles per gallon" figure then you should multiply by 148 which gives your initial figure of 100 passenger miles per gallon.
Now, the average modern family car does 40mpg on the motorway and can carry up to 4 passengers. This is 160 passenger miles per gallon which is a higher figure is it not?!