"Fuel Efficiency Devices”FUEL additives and retrofit "efficiency” devices are many of the so called off-shelf items designed and sold by many third party car care persons and shops. 

There are several, perhaps hundreds of companies out there all claiming to manufacture devices which give better fuel economy, cut down on emissions, add to engine longevity etc. For the most part, these products are elaborate cons. And in some cases, not-so-elaborate. For as long as people are gullible, and can be led by advertising hype, these companies will thrive.  What you need to consider is one very important question: If these devices work, why do the manufacturers not put them on their cars as standard? Surely selling a Subaru equipped with a device that gives 8% better economy than the equivalent Nissan would be a killer sales pitch? Well the answer is blindingly simple: these devices don’t work as advertised. There’s an important distinction there because a lot of these devices do something but just not necessarily to the level promoted in the advertising.MPG Caps or any of the other "pill in your fuel tank” products are the cheapest and easiest-to-find miracle cures for increased power and gas mileage.

The products are simple. They’re normally pills, in a pop-out wrapper just like medical pills, or in a bottle. They smell foul and the manufacturers claim that one pill per 20 gallons of fuel will give you increased power and performance whilst at the same time cutting your mpg (in some cases they claim up to 30%). Explanations range from "organic engine conditioner that improves fuel economy and power by creating a micro-thin coating on the combustion chamber allowing fuel to burn better” all the way up to the fantastical "nanotechnology particles that seek out and change the molecular structure of refined petrol to make it burn more efficiently”. Wow. Impressive stuff. Typically these pills are biodegradable plant or sugar compounds laced with food colouring and some odd petrochemical to give them an official smell. They don’t work. ABC News reported on a AAA (American Automobile Association) test of the MPG Caps and found their claims to be invalid. Texas closed down Bio-performance Inc. for fraud linked to their magic fuel pill.  The MPT Smog Buster is yet another one, it is a device projected "holographic frequencies into the fuel tank and changes the molecular structure of the petrol” what would you think? Well it seems that plenty of people think enough of that statement to cough up $299 for a plastic disc with some fancy silk-screen printing on it. The idea is, apparently, that the molecularly-changed fuel burns more efficiently thus giving you better fuel mileage and fewer emissions. It’s about the size of a coin and is supposed to - wait for it - be taped to the underside of your fuel tank. Its vendors through their website even helpfully suggest that superglue could be used.  "It doesn’t work,” says Dr. Terry Parker, a physics professor at the Colorado School of Mines. It was tested for 9News. "It’s clear that it’s just a sticker and nothing else,” the expert said. What should set alarm bells ringing with some of these devices is that they are sold through multi-level marketing, the new buzzword for pyramid selling.

The more you sell of them, the cheaper they get for you to buy. The best part of the sales pitch is this section; some of them like Ocean City Network, Inc. make offers like, we are proud to offer a 30 day 100% money back guarantee if not completely satisfied. To qualify for a full refund, the MPT SmogBuster must remain adhered to the paper in which it was shipped on. So if you un-stick it and use it, you can’t get your money back.

However, they add that to test it, you can simply duct-tape the thing to the gas tank, complete with its backing paper. So even if the device was electrical (which the silk-screen printing might lead you to believe), putting a piece of paper in the way would prevent it from making any contact with the metal of your tank.

I also suspect that trying to remove the duct tape from the fuel disc without removing the fuel disc from the backing paper is going to be next to impossible.