A gallon of gas has 30,000 Calories. An SUV weighs (technically masses) 2000kg and gets 20 MPG. Thus the kilogram miles per Calorie -- the efficiency -- for an SUV is 1.33.

A peanut butter sandwich is 600 Calories (with bananas and maple syrup on whole wheat, but corn syrup on Wonder bread is similar). A bicycle plus messenger bag and rider weighs 80kg and gets 10 miles to the peanut butter sandwich. Thus the efficiency of the bicycle is 1.33 kg.mi/C.

So far the bicycle and SUV seem exactly equal in efficiency and have equal environmental impact.

But that doesn't take into account cost. The peanut butter sandwich costs $5, or $3 if you make it at home. Call it $4. The gallon of gasoline also costs $4. But the gasoline has 50x the energy. The total cost to society is 50x higher to operate the bicycle, taking all process costs into account through the price system.

So in fact, biking is terrible for the environment. We'd all be better off banning bicycles and buying every cyclist an SUV and then having a mass lycra bonfire. Plus, think of all the cash we would save by not blowing it on extra bicycle fuel. It could save the economy.