The EPA and other labs have tested many of these "fuel saving" devices and found them to be totally worthless. At best they do nothing. Some actually reduce power and fuel economy and/or increase tailpipe emissions.
So forget about throttle body spacers, tornados, turbonators, vortex swirlers, fuel line magnets and the like. If an automotive engineer could add a cheap and simple device like these and improve fuel ecomomy by even 0.2 MPG, he would be worshipped as a minor god. Even the most diligent consumers would be hard pressed to notice a tiny improvement like that. And, yet, the scam artists continue to promise a 10/20/30% increase in power and/or economy. Yeah, right...
Maintain your vehicle, slow down, and remove excess weight for the easy improvements. If you are looking for worthwhile mods, invest in something that reduces parasitic losses like an E-fan or an underdrive pulley. If you drive mostly highway, a hard tonneau might
buy you an MPG or two.
Here's some good reading that shows these "magic" devices to be either worthless or actually detrimental to power, economy and/or emissions.