Fuel additive compositions include a plurality of metal nanoparticles and a carrier that is dispersible in a hydrocarbon fuel. The metal nanoparticles can be spherical-shaped and/or coral-shaped metal nanoparticles. The carrier can be liquid, gel or solid and can be readily miscible or soluble in a hydrocarbon fuel such as gasoline, diesel, jet fuel, or fuel oil. The carrier can be a solid carrier configured to allow the hydrocarbon fuel to dissolve the solid carrier in order to release and disperse the metal nanoparticles within the hydrocarbon fuel.