Products of this type include: Acetone, PowerPill, BioPerformance, Ethos FR, FFI MPG-Cap,
Motor Latte, PowerPlusMPG, EcoEnergizer

Unlike the great majority of the products mentioned on this site, which are generally "fit and forget", there are also a number of products intended to be added to each and every tank of fuel, at a cost that may be several percent of the cost of the petrol (gasoline) or diesel they are added to. Recently these type of products seem to be increasingly common - perhaps because so many "bolt-on" devices have been exposed as scams, perhaps because the complexity of modern cars makes motorists reluctant to "tinker under the bonnet (hood)".

Many of these products are of course intended to meet genuine needs. For example, old engines may require a valve seat protector when used with unleaded fuel, or very high performance engines may require an octane enhancer when "ordinary" petrol is used. I would generally prefer to buy a "pre-mixed" fuel that already meets these requirements, rather than trust a possibly sub-standard additive, but there are certainly plenty of good quality and effective additives available for this kind of work (please do not ask me to recommend one, though). Another potentially useful product is an additional stabiliser, for vehicles that are only used infrequently and where the fuel could otherwise "go off".

Where I am far more sceptical is with additives that claim to enhance the combustion process in some way. Typically these supposedly make the fuel vaporise better, burn more quickly or more completely (a catalytic effect), or clean deposits. In all these cases, I am highly doubtful that large benefits can really be obtained in this way, as you will see from the links above. Especially critical is that using these additives may add 5 or even 10% to the cost of a tankful of fuel, so unless very large improvements in economy can be obtained, the overall effect is an increase in cost. Although the products vary greatly in their supposed effects, two particular claims are very common:

An engine cleaning effect. As explained here, most modern engines should not need additional cleaners unless they have been run on especially poor quality fuel. (The very few cars that are in need of cleaning may be responsible for some of the large gains occasionally seen with these products.) But if you think your engine does need cleaning, why not just use a product specifically sold and marketed as a cleaner, or indeed run your car on top-quality fuel (with a high detergent content) for a couple of months?

A more complete / faster burn. Typically makers of these product suggest that a large proportion - 10% or more - of the input fuel escapes unburnt from the engine, to be either released into the atmosphere as pollution or uselessly burnt in the catalytic converter. This is just not true, at least for any reasonably modern car in good condition - the true loss is only about one or two percent, so the potential for improved economy is equally small. A related claim is that the burn is "faster" or "hotter"; without evidence, I am sceptical that this enhancement in burn rate really takes place, but in any case the critical point is that the engine is already optimised (ignition timing, etc) for the "normal" burn rate. A faster burn, even if theoretically beneficial, is likely to make economy worse unless engine parameters are optimised to suit. For further discussion on this point, see this page.

Especially suspicious to me is that virtually every product I have come across claims to be equally effective on petrol (gasoline) and diesel engines, which given the radically different combustion processes strikes me as unlikely.

It's also worth pointing out that many, if not most, of these products are sold through Multi-Level Marketing, which immediately triggers warning bells among sceptics. Some perfectly good and effective products are sold through MLM, but it is also a common method employed by scammers and snake-oil salesmen to maximise revenue from ineffective products. (Commonly, the "early adopters" make much, if not most, of their money by recruiting lower-level sales people rather than directly through product sales.) This sales method also means that there is an army of thousands of bottom-level sales people, all with a financial stake in the success of the product, who will pop up whenever it is discussed on Forums and newsgroups and say how marvellous their product is or write glowing "independent" reviews of it. Some, or even most, of these people do genuinely believe in the product, but their lack of financial independence means that any testimonials you see should be taken with an even larger pinch of salt than normal.

A particular concern with some of these MLM companies is their refusal to accept credit card payments for product purchase or sign-up charges. There may be genuine reasons for this, such as the difficulty of setting up such systems, but it seems many companies tell customers for months on end "credit card payments are coming any day now". The suspicion of sceptics is that card payments are deliberately refused, since these usually give customers the right to demand their money back from the credit card company if the product turns out to be worthless or the seller goes bust. So it may be that the credit card companies think the idea is a scam, and don't want to be involved, or else the people running the MLM company are scared that the massive legal power of Amex, Visa, etc will come after them if it all goes wrong.

An additional problem with MLM products is the "Chinese whispers" effect - a company may make quite modest claims for their product (say up to 5% economy gain) but as the message travels down through the "pyramid", the claims become more and more exaggerated. By the end, some people selling the product to end customers may be claiming 50% economy gain. Or a company may mention that the product is being tested on a small scale by a particular institution, which somehow turns into an endorsement of the product by that institution. This isn't the fault of the selling company, of course, though some do seem reluctant to correct the misconceptions.

For more information on some additives that are currently widely discussed on the Internet, see the Case Studies on:
- Acetone
- BioPerformance Gas Pill
- Ethos Fuel Reformulator
- Fuel Freedom International's MPG-Caps
- Extreme Research's EnviroMax Plus

(Some of these products are claimed to be "not fuel additives", but I am using the term in the sense of "something you add to the fuel", rather than any technical definition that may exist.)

For the avoidance of doubt, I am equally sceptical about all the other fuel "saving" additives I have come across, but do not have the time to write complete critiques of all of them. Please look at the general comments, or read this page again, to understand my concerns about all products of this type.

Please also read the general comments on fuel "saving" devices, if you have not done so already

If you found this page helpful, you may like to support my work. If you think I have made a mistake, or am talking complete nonsense, please take a moment to read the Response to Critics before saying so in public (on discussion Forums and the like.)