The price of gas keeps going up and so does the amount of untried and untested items that are professed to get you increased fuel economy. I've been involved with making cars go longer and stronger, on less and less, since 1967 and have seen lots of different ideas. Some work and others don't. It's a case of upsides and downsides.
One downside is, you may find the item only works in the inventor's or salesperson's mind and has no foundation in fact. The good thing is there are items that are low-tech, have been proven to save fuel, increase power, lower emissions, keep the engine clean and are installed on possibly thousands of vehicles: