These devices, like fuel ionization systems, allegedly prepare the fuel to burn better when it reaches the combustion chamber. Rather than using electric fields to do the job, however, fuel-line magnets claim to use powerful magnetic fields to break down fuel into its basic components.
As is the case with fuel ionizers, fuel-line magnets serve little purpose other than to separate uninformed drivers from their money. Petroleum fuels are ubiquitous because of their stability -- although they can't pack the same punch as fuel sources like hydrogen, they're much safer and easier to handle. Petroleum fuels, in fact, are too stable to be significantly altered by something as small and simple as a magnet placed on a fuel line. Even if a magnet could produce a significant electrical field, that field would be altered by the metal of the fuel line, tank and components. If anything, an extremely strong magnet might disrupt some of the car's more sensitive electronics, although even this is unlikely, given the careful engineering that has gone into today's reliable electronic components [source: Allen].