Popular Mechanics and Mythbusters proved that most "Fuel savings" devices are hoaxes

Here at PM’s automotive department, we’ve debunked most of the devices on the market today that claim to increase your fuel economy. They simply don’t work. And in some cases we’ve found they can actually make your vehicle run worse.
http://www.popularmechanics.com/blogs/automotive_news/4199963.html

The show also noted that out of 104 fuel efficiency devices tested by the EPA, only seven showed any improvement in efficiency, and even then, the improvement was never more than six percent. http://en.wikipedia.org/wiki/Fuel_saving_devices

In summary, if you want to save more fuel, increase the efficiency of your vehicle, use less gas to drive the same distances there are only 3 proven ways to do that.
1) drive only on correctly inflated tires, less that the recommended pressure is proven to be bad, and overinflating is dangerous, makes your handling worse, and is proven to not work by the Mythbusters
2) drive like you are out to impress the most difficult critic, as if every thing you do is imporatnt to save gas by not accelerating hard, not staying on the gas when approaching a stop light that is red, coast instead.
3) drive a vehicle that is proven to get better gas mileage

Also http://gas2.org/2008/05/29/save-gas-without-losing-your-shirt-3-gas-saving-devices-with-high-scam-potential/