This is what's been on my mind lately: Passenger miles per gallon.
A cyclist gets 600mpg, and a person driving a car gets 40mpg. Well, what about with 2 people? Lets say a car gets 40mpg. With one person in the car, that's 40 passenger-miles per gallon. With two people in the car, it would achieve 80 passenger-miles per gallon (move two people 40 miles with one gallon). Four-up, it would get 160 passenger-miles per gallon (4*40mpg). More on the bike later.
This concept is especially important when calculating the environmental impact per person of mass transit. A bus, which gets an abysimmal 4mpg, gets 160 passenger miles per gallon with 40 passengers on board (40*4mpg). The trouble is, if a bus holds 40 people, and you want to move 41 people, you need two buses. you double the amount of fuel consumed to move that extra more person, and the passenger-miles per gallon of the convoy comes down to 81. (from: 41 passengers*(4mpg)2 vehicles)
A 75% full 747 might get 30 passenger-miles per gallon, and a cruise ship gets about 10-passenger miles per gallon.
So what about a bike. I got 633 miles per gallon, but what if two people go on a bike ride. It takes twice as much energy for two people to ride bikes, but they get twice as many "passenger-miles." So the efficiency stays the same. So whether one person rides to work, or everyone rides to work, each person is getting 600 passenger-miles per gallon [ 2*(600 mpg / 2 vehicles) ]. And you don't have to go in the same direction as anyone else!
I saw a guy with a kid in a child seat on the back of his bike on my way to work this morning and thought, holy crap, (approx.) 1200 passenger miles per gallon!