From How Stuff Works:
Is there a way to compare a human being to an engine in terms of efficiency?
It turns out that “biological engines” — which is what the muscles in your body are — are pretty amazing in terms of efficiency. To find out how efficient, let’s look at how many calories a person burns while riding a bicycle.
If you look at a page like this calorie chart, you will find that a person riding a bicycle at 15 miles per hour (24 km per hour) burns 0.049 calories per pound per minute. So a 175-pound (77-kg) person burns 515 calories in an hour, or about 34 calories per mile (about 21 calories per km).
A gallon of gasoline (about 4 liters) contains about 31,000 calories. If a person could drink gasoline, then a person could ride about 912 miles on a gallon of gas (about 360 km per liter). Considering that a normal car gets about 30 miles per gallon, that’s pretty impressive!
To be fair, keep in mind that a car generally weighs a ton or more, while a bicycle weighs only 30 pounds. Cars also travel a lot faster than 15 mph. But it is still an interesting comparison. Note also that people cannot drink gasoline. However, people can drink vegetable oil, which contains nearly the same number of calories per gallon (if you look at How Fats Work you can see that fat contains long hydrogen/carbon chains just like gasoline does).
The people riding in a race like the Tour de France are riding more like 25 mph. Because air resistance rises very quickly with speed, they are burning about three times more calories — something like 100 calories per mile. In a 100-mile stage of the tour, a racer might burn something like 8,000 to 10,000 calories in one day! So they are getting only about 300 miles per gallon. The only way to replace those calories is to eat a lot of food (see How Dieting Works for details).