Fuel is a complex mixture of hydrocarbons, and the magnetic properties of these compounds are very weak. The magnetic fields generated by the magnets used on fuel lines are too weak to have any significant effect on the fuel's molecular structure or its combustion properties.
In addition, the flow of fuel through a fuel line is typically turbulent, which means that the fuel molecules are constantly colliding with each other and the walls of the line. This turbulent flow disrupts any magnetic alignment that the magnets might create, preventing the magnets from having a consistent or sustained effect on the fuel.
Therefore, while some individuals may claim that magnets on fuel lines improve fuel economy or engine performance, there is no scientific evidence to support these claims. The use of magnets on fuel lines is generally considered to be ineffective and a waste of money.