Thats not what the magnets are advertised to do. The two magnets wrap around the fuel line itself, and in doing so creates a magnetic force that supposedly breaks up large hydrocarbon molecules, that multiplies clumped molecules in to simple, single easy burning molecules. I have used them on my cars for years. Its one of those things that you buy that you say to yourself, "Ahh its only $20, might as well as try it, what have I got to lose." No it didn't provide any noticeable mpg inprovements, similiar to the Tornado that goes inside the intake, metal, air disturbance, maker, or the throttle body or Carb spacer that supposed to give the gas air fuel mixture more time to agitate and mix together before going into the combustion chamber. What about the foam air cleaner elements? I've got all four addons on my Suburban, maybe all of them combined I got 1 mpg more. It was worth the $230.
i dont believe that would do anything, maybe stop metals in the fuel from getting to the engine, but that might make a bigger problem down the road if it causes blockage in the line.