With the price of gas what it is today, we're all looking for a little more MPG. It turns out, back in 1973/4, during the first oil crisis the US experienced, Car + Driver magazine ran an article featuring $11 worth of modifications to a Ford Pinto, plus some radial tires and got a whopping 25% better fuel economy. How so? They focused their attention on minimizing the work the engine had to do (which was mostly pushing against air- also called aerodynamic drag). The next time you complain about gas prices, check out how aerodynamic your vehicle is. If you have no clue, I'll give you a hint: round shapes with minimal surface area (bullets) are highly aerodynamic; odd shapes with lots of surface area (tractor-trailers) are not very aerodynamic. Check out what this aerodynamicist has to say about this stuff.
Also check out this video of a home-made automatic truck spoiler that someone devised from their tailgate. The strips of ribbon let you know when air is pushing against or flowing over the tailgate. When the air is flowing over the tailgate, as opposed to being caught in the truck bed pushing against the tailgate, it is reducing the aerodynamic drag and increasing the fuel efficiency of the truck.