The human body can be compared to a bicycle in terms of engines. The human body's biological engine is it's muscles. A person riding a bicycle at 15 miles per hour burns 0.049 calories per pound per minute. So a 175 pound person burns 515 calories in an hour, or about 34 calories per mile. A gallon of gasoline contains about 31,000 calories.
If a person could drink gasoline, then a person could ride about 912 miles on a gallon of gas. Considering that a normal car gets about 30 miles per gallon, that is pretty impressive results. Of course, people can't drink gasoline and a car weighs about a ton while a bike weighs about 30 pounds. We can, however, vegetable oil which has the same amount of calories as gasoline. Professional bikers get more than 25 miles per hour, too. So, it is a decent comparison of humans to bicycles.