Pretty simple if you think about it...

1) Buy brewing ingredients at a local homebrew shop.
2) Brew delicious homebrew.
3) Keg or bottle delicious homebrew.
4) After getting home from hard day of work, have a homebrew.
5) Have another... and perhaps one more.
6) Become legally impaired, and thus unable to operate a vehicle safely.
7) Leave car in garage for the evening, foregoing trips to dry cleaners,
    convenience stores and restaurants - all places where you would just
    spend more money anyway. Cut gas expense by 50%.
8) Funnel savings into making more delicious homebrew, offsetting the cost
    of increased brewing volume.
9) Law of supply and demand kicks in: Lower demand for gas = Lower prices.
10) Save money... save gas... save the world!

Just goes to show you... if only the world had more homebrewers, we might
    not be in this mess...