It's true that bike commuting is much cheaper then driving. For me the biggest expense is food/nutrition. I not only commute but I also do long distance rides on the weekends, and that means I need to eat more then an average person who is not as physically active as I am. If I don't eat enough nutritious food then my body would never be able to keep up. So food is the biggest expense, and I would much rather spend my money on good food then on gas.