The cheapest and most effective fuel economy device you can get is an egg. Put it between your foot and the gas pedal before you start out on a trip and try not to break the egg. Seriously though, the most effective item for fuel economy on any given vehicle is the driver! I have seen statistics suggesting that the average American could gain 20% in fuel economy if they were just to change driving habits.

I also recall reading an article on "gas saving" widgets such as water injectors and the "turbines" you mention. BTW, they were available when Model A Fords were new; the same claims were made then. The conclusion was that any fuel savings were due to changes in the way the driver drove after the device was installed. Also, there was seldom any data on fuel mileage before the device was installed so any "improvement" could not be rigorously documented.