Fuel magnets are marketed as a kind of fuel catalyst and are supposed to provide all sorts of benefits.
My knowledge of physics (chemistry being just a subset of physics really) is pretty strong and I struggle to understand how magnets attached to your fuel line can do anything with the fuel and its combustion properties.
OK, if your fuel is laced with microscopic particles of ferrous metals then it will attract those particles thus preventing them from entering the engine's fuel system. See my post on oil filter magnets - these work for exactly that reason.
I cannot accomodate the idea that there are any magnetically chargeable arrays of atomic particles in ANY fuel, diesel or petrol, whether fossil or vegetable derived.
So, how can a pair of magnets, however intense the magnetic field, have any effect upon the engine's use of fuel?
OK, let's say you have a car that's been abused and under-serviced for 50,000 miles. You then choose to give it a thorough 'work-out'. Flush the oil, change the filters, drain the tank of stale fuel, chuck injector cleaner through it, new plugs and spark leads, discs and pads all round, sort the tyre pressures and get a full four wheel alignment, new brake fluid etc. and also fit a fuel magnet, then of course it's going to drive better and give better economy.
But this is not the result of the fuel magnet.
Fossil or vegetable derived fuels are not in any way sensitive to magnetic static fields.
If the field was alternating then you could argue a minor case because this would slightly warm the fuel (because it slightly warms the metal fuel pipes) and might JUST, if you are lucky, with a diesel engine, allow the injectors to have a slightly easier time of injecting the slightly less viscous fuel.
But, for me. Nope.