...And you can do so by not buying one. I should explain, that these devices, usually a magnet (or three) which is clamped or tied around the fuel line of your vehicle, is
NOT CAPABLE OF DOING ANYTHING, ANYTHING AT ALL, TO YOUR FUEL.
If you want the scientific explanation, take a GCSE in Chemistry, and pass it. Just this basic qualification (I'm rather further on than this, by the way) will provide enough education for you to realise that the psuedo-science that is used in the listings of these products, is Grade-A Bull-Plop. The 'inconvenient truth', is that...
1) Magnets are incapable of ionising any liquid. FACT.
2) Fossil fuels are COVALENTLY-BONDED compounds. Not ionic ones. Not capable of being ionised. GOT THAT?
3) Petrol, and diesel, are both covalently-bonded carbon compunds, and as such are COMPLETELY UNNAFFECTED by magnetic fields. Take a litre of your fuel, place it under a scrapyard car-lifting magnet, from now until next Christmas, and IT WON'T AFFECT THE FUEL, NOT ONE TINY LITTLE BIT, NOT AT ALL, IS THAT CLEAR?
3) Covalently bonded molecules - which is what the fuel is - are not ionic. Molecules are either ionic or covalently bonded. Carbon chemistry - fossil fuels - are all COVALENTLY BONDED. It would be lovely if there WAS a special little fizzy electronic charge we COULD put onto a fuel molecule, to make it burn more enthusiastically, but SADLY THERE ISN'T. Oh, and point 1) applies, anyway. Point 1), people? MAGNETS ARE INCAPABLE OF IONISING ANY LIQUID.
4) EEEEEEEEEEEEEE-Even if it did work, which it doesn't, just consider the journey of a fuel molecule from the tank, through the fuel-filter, through the pump (boy, we are really moving now) and being shot at high pressure - around 80 -120 PSI, down this narrow hose, at great speed. It would pass the magnet in about a ten-thousandth of a second.... Not very long for our MAGIC to work, is it? Oh, sorry, there is no MAGIC at work here anyway.
5 Rubber fuel lines. That's what vehicles have. These listings show the magnets attached to the rubber fuel lines. Get a magnet, place it under paper, and put iron filings on top . Pull the magnet around, and see the iron filings move, you've done that before right? Try that with a rubber sheet. It doesn't work, because the paper was magnetically transparent, but the thin rubber sheet has inhibited the magnetic field. And the rubber? That's a carbon-based, long-chain, covalently-bonded molecule, in the same family as the fuel that you are trying to affect somehow with the magnet. The rubber sheet was unafffected. But it did block the magnetic field, perfectly. Many, many substances are completely unnaffected by magnets. Like water. Wool, leather, wood, rubber, petrol, diesel, aluminium and hell, pretty much everything but iron. Try plastic. No change. Cheese. Unaffected. Damn. What about bacon? Yep, no change from zapping it with a magnetic field. Just like petrol and diesel then.
6 So, to clear this up, these magnets 'work', by putting a field, (that we can show doesn't even make it through the fuel-line) into an organic molecule, (that they can't affect) in order to 'ionise' the molecules, (which no magnetic field can do anyway) for the briefest instant, as the molecule zips along the fuel-line in the 'blink-of-the-blink' of an eye.
We know a lot about how to get good combustion in the cylinder. High compression. Good inlet-charge flow through the inlet manifold, around the port (good 'swirl' characteristics) and good turbulence ('squish') around the inlet valves as the charge of fuel/air vapour flows into the combustion chamber. This is all in the hands of your cylinder-head designer. Then, we need a high-energy spark, delivered at the right time. Better yet, sparks from two plugs, as per Alfa Romeo's TwinSpark engines, and the latest large engines from Range Rover and Mercedes. If you want to save on fuel bills, increase efficiency, and get the best from your engine, you can service and tune it. Empty the boot of the junk you are carrying, check your tyre pressures, and drive like you are trying to save fuel.
Don't waste your money on these useless, bogus products - they will do nothing.
You could ask the seller to show you the INDEPENDANT LABORATORY TEST REPORTS on their magnet fuel-saver products. Just don't hold your breath.