You have only to surf the Internet for a short time to find any number of conspiracy theorists, convinced that the car industry knows of all kinds of fuel saving technologies but has actively suppressed them due to pressure from oil companies or "shadow governments". Chief among these are Charles Pogue's "200 mpg carburettor" (a device to improve atomisation / vaporisation) and various engines allegedly running on water.

The great majority of these supposedly suppressed devices are of course total myths - either the device never did exist, or it did but the benefits were very much less than claimed. While it is tempting to believe the car industry is in the pay of oil companies, just consider some of these facts:

• How could Communist-era Russian and Chinese car companies be controlled by the oil industry?

• Why would governments in countries such as Japan (which must import almost all the oil they use, and so are desperate to reduce consumption) collude in this?

• Why has no proof ever leaked out, given the vast sums of money that must be involved? (After all, the tobacco industry couldn't keep the cancer link secret, nor could Ford hide the Pinto scandal; in both cases we're talking about big, powerful companies)

• Why are manufacturers spending billions of pounds on developing diesels, hybrids, fuel cells, etc, if they have no interest in fuel economy?

• Indeed, if oil companies (or other shadowy background organisations) are actively suppressing fuel-saving technologies, why are they allowing development of diesel and hybrid vehicles - which undoubtedly use significantly less fuel than conventional petrol engines? How can such vehicles, giving fuel savings of 20% or more, be made in vast quantities and sold all over the world, if the "powers that be" are willing to assault or even murder individuals making such claims?

What is however unquestionably true is that the political and legislative environment in the US has not strongly encouraged consumers to choose more economical vehicles; indeed, the different tax and legislative treatment of cars and "light trucks" has acted, whether by accident or by design, as an incentive to do the opposite. As a result the US car industry, for obvious and entirely sensible business reasons, has not generally seen development of such vehicles as a high priority. The reasons for this are partly historical, but those who note close links between the current US Government and oil companies (both in the US and abroad) may have a point. However, this is very different to a worldwide conspiracy that actively suppresses fuel saving technologies through violence and intimidation.

Any in any case, things are rapidly changing in the US. Thanks to rapidly-rising fuel prices, consumers are switching in droves from more thirsty vehicles (typically SUVs and pickups from US manufacturers) to those that get better gas mileage (typically small and medium saloon cars from Europe and Japan). Partly as a result, the big American manufacturers are losing sales and profits - for example, GM lost over ten billion dollars in 2005. Yet the conspiracy theorists believe Ford and GM have "miracle" gas-saving technologies under their control, which would greatly increase profits at a time when they are sacking workers and closing factories due to huge financial pressures - but they keep these technologies hidden because their friends in the oil companies ask them to. Does that seem at all plausible?

A related claim is that the car makers, possibly in collusion with oil companies, deliberately set out to make their vehicles as prone to breakdown / wearing-out as possible, in order to make more money from servicing. While there is some logic to this, in practice it seems highly unlikely to me, because the intense competition in the industry means that any company offering more durable vehicles (especially if backed up by a long warranty) can charge a higher price and so make more profit. Also, major emissions-related components are usually warranted up to 7 or 8 years by legislation - see for example warranties from Ford and Toyota. Given that the car makers must pay for any failures in these components, why would they want to make them wear out or break more quickly?

An interesting aspect of many (though to be fair, not all) of these "conspiracy" websites is their attitude to other energy saving ideas. They usually say how there is this terrible conspiracy and we should all cut down our fuel use to punish governments and oil companies. Of course, their site exists purely to help others, save the planet, and cut the profits of greedy oil companies. It seems odd, then, that they entirely ignore quite genuine fuel-saving ideas that are either free or at least relatively cheap, instead concentrating on selling their miracle gizmo (which of course you must buy quickly before the bad guys close them down.)

Anyone who follows the advice given here, here and here on my site is virtually guaranteed to cut their fuel use by 25 - 50%. Yet despite this advice (and indeed the criticism of oil companies I make here), I have not received a single death threat; no large men with dark glasses and suspicious bulges under their jackets are lurking outside my house! (And indeed, during the eight years I spent developing more fuel-efficient engines at Rover/BMW, not once did anybody "warn me off" a particular technology.)

The truth is that the stories of people being murdered, and their miracle inventions stolen and concealed, are almost certainly just urban myths. In some cases sellers of fuel "saving" devices are pursued by the authorities, for the simple reason that they are committing criminal fraud and quite rightly should be fined or imprisoned for effectively stealing large sums of money from the public.

One wonders if some of the more extreme stories are in fact invented by fraudulent sellers of fuel "saving" devices. If you have taken tens of thousands of pounds from investors / buyers for a product which either does not exist, or does exist but is of no significant benefit, then the "evil crooks from the oil companies stole my invention" excuse might well be very appealing.

Having said that, for those who absolutely insist they have personally seen inventions stolen, there is one possible explanation. Suppose a car maker heard of a potentially very useful invention (such as a very economical engine) but the inventor either refused to talk to them, or demanded a huge up-front payment. Seeing the chance to make tens of millions of pounds (dollars), a sufficiently ruthless company might perhaps decide to simply copy the idea without properly compensating the inventor, and so would steal the prototype vehicle to investigate it. If the idea turned out to be genuinely useful, then it (or some part of it) would be quietly added to production cars - if not, it would be filed in the "nice idea but..." pile. I could - just - believe that some car makers would be ruthless enough to steal and copy an idea that would make them a fortune, but I can't believe they would be stupid enough to simply supress it.

On a similar theme, people often claim that the patent has been "bought up and concealed". Yet Patents are available in hundreds of libraries throughout the world, and on the Internet - for example, here is the "secret" Patent for the Pogue carburettor, freely available to anyone who wants to make one. And patents expire after 25 years, so anything "supressed" before 1980 can now be made by anyone without restriction. Finally, remember that you don't need to prove something works in order to patent it - there are plenty of patents for perpetual motion machines, but that doesn't prove they really exist.

Another interesting "conspiracy" theme is the addition of ethanol (alcohol) to petrol (gasoline), especially in the USA. The conspiracy theorists say that adding ethanol makes fuel consumption worse, and imply this is part of a plot to increase oil usage.

In one sense, this is quite right, but no mysterious "anti-economy" properties are required to explain this. Quite simply, ethanol has not much more than half the energy content of gasoline, and so a 10% ethanol mix will give (typically) about 3-4% worse economy. But ethanol has never been promoted as an economy improver; rather, it is added for two extremely good reasons, that have nothing at all to do with fuel consumption.

Firstly - and the original reason - is to reduce toxic emissions. Since it contains a large amount of oxygen, it slightly weakens the mixture on cars without oxygen (lambda) sensors, hence reducing CO and HC emissions. Additionally, it acts to increase octane rating (and so suppress knock), while being rather less toxic than many other substances that have this effect - for example benzene.

Secondly, and the reason why it is now of increasing interest, it replaces oil-based fuel with biomass-based fuel; countries that use it can grow their fuel instead of importing it (perhaps from unstable countries in the Middle East), and the CO2 produced when it is burnt is balanced by the carbon taken in by the plants it is derived from. (Some people suggest that the energy used in making ethanol (harvesting crops, processing, etc) is actually more than you get back when it is burnt. Possibly so, but the principle is certainly sound.)

Car makers, as a rule, would prefer it if petrol (gasoline) did not contain large amounts of ethanol. In concentrations beyond a few percent, it is highly corrosive to fuel system components (notably aluminium). Making vehicles able to withstand ethanol actually adds significantly to the cost of manufacturing, but the makers have to put up with this due to pressure from governments and consumers to be able to use high-alcohol fuel.

Many conspiracy theorists nowadays regard this website, and me personally, as part of the conspiracy - a "paid stooge of the oil companies". Seemingly I sit on my luxury yacht in Monaco drinking Dom Perignon and generally enjoying a playboy lifestyle; every so often somebody from Shell, Ford or whatever comes along with a suitcase full of money and says something like, "The Super FuelMiser 9000 really works - quick, say something bad about it on your website, before people start using it".

All I can say to that is...I wish! The only income I get for this site is the PayPal donations, and they barely cover the hosting charges.

One fuel-saving technology that undoubtedly has been supressed - but for very good reasons - is lean-burn. The ratio of air to fuel is a critical factor in petrol combustion. The theoretically ideal mixture is around 14.5 parts of air to 1 of fuel. This is known as the stoichiometric ratio. Variations from this are characterised by the term lambda, where lambda less than 1 means a rich mixture (excess fuel), and lambda greater than 1 means a lean mixture (excess air).

All modern petrol engines run at lambda = 1 under most conditions, but it is well known that significant economy benefits can be gained by running the engine much leaner - ie, with excess air. To obtain the maximum benefit the engine must be optimised in terms of combustion chamber design, ignition timing, etc, so as to allow operation at lambda = 1.7 (24:1 AFR) or even leaner. Various engines to do this were developed in the 1980s (for example Honda's VTEC-e), and could be up to 10% more economical than more conventional engines.

However, in 1992 all these engines disappeared from the European market (and from the US market a few years earlier) with the adoption of the "Euro 1" vehicle emissions standards. Many people see this as "proof" that a conspiracy between oil companies, governments and car makers acted to supress the technology and so force people to use more petrol.

In fact the truth is much simpler, and can be summed up in one word: NOx. NOx (oxides of nitrogen) is one of the three main toxic pollutants from car exhausts, and by some measures is the most dangerous of the three. Estimates of the number of deaths per year in Europe through NOx pollution range from a few hundred to a few thousand. For obvious public health reasons, European legislators wanted to reduce emissions of NOx and so imposed strict limits to apply from 1992 onwards. (NOx is formed when oxygen and nitrogen in the air combine at the high temperatures that inevitably occur during combustion, and is entirely unrelated to the presence of any nitrogen contaminants in the original petrol.)

Unfortunately it was completely impossible to meet these limits through lean-burn technology. Lean-running engines produce less NOx at source than engines running at lambda = 1, but cannot use a three-way catalytic converter to remove the NOx. (What the catalyst does is to remove the oxygen from the NOx and give it to the HC and CO. The result is plain nitrogen, carbon dioxide and water, all of which are essentially harmless. But to work, the lambda value must be just right. If the mixture is too lean then the "spare" oxygen is given to the HC and CO, and the NOx passes through unconverted.)

By contrast, the catalytic converter on a "stoichiometric" engine can convert 95-plus percent of the NOx in the exhaust to harmless nitrogen and oxygen, and so achieve much lower tailpipe emissions levels. I personally worked on lean-burn engines for many years, and a lot of companies spent hundreds of millions of pounds on developing the technology, but in the end the NOx emissions were always at least ten times higher than for a catalysed engine. The danger to public health from these emissions was, rightly in my view, seen as a price that was not worth paying for better economy. Aftermarket modifications to cars that may give fractionally better economy at the expense of vastly increased NOx (eg air bleed devices) are specifically banned in the US, with large fines imposed on anybody who "tampers" with their car in this way - hopefully it is now clear that this is for sound public health reasons, not because of an oil company conspiracy.

By the way, claims that the catalyst does not start working until the car has been driven five miles or more are entirely wrong. The standard European emissions test is less than seven miles, and the catalyst is working for the vast majority of this. On the most recent cars, the catalyst is active within a minute of starting the engine. A minute after that, it is converting 99% or more of the toxic pollutants to harmless gasses - the emissions with the "cat" are only a hundredth of emissions without it.

It is also not true that catalysts "die" after just a few tens of thousands of miles. Car makers must certify that their vehicles still meet the strict emissions standards (meaning that the cat must still be working efficiently) after 100 000 miles of normal driving. In a few rare cases catalysts do fail before this, but on any vehicles sold in Europe after 2000 (or in the USA after about 1997) this is automatically detected by the engine management system and a warning light on the dash illuminated.

The catalytic converter (which is poisoned by lead) is also the primary reason why unleaded petrol was introduced, although there undoubtedly are health issues with lead as well. Many people observe that unleaded petrol, with its high benzene content, is pretty nasty stuff as well - it is, but because the cat does such a good job of cleaning up the unburnt hydrocarbons in the exhaust we are still better off than with leaded petrol and no cat.

The cat (and associated lambda sensor) are of course frequently cursed by owners of older cars faced with large repair bills. This is quite understandable, but it is well known that a very small proportion of highly-emitting cars generate most of the toxic pollution in our environment. To achieve the same clean-up, you could either spend £500 on repairing one old car, or add £50 of extra technology to 100 new cars. The former is clearly more cost-effective.

However, despite all the above, lean-burn is not totally dead. Stratified-charge direct-injection engines (such as Audi's FSI and Ford's SCi) use an extremely lean mixture with fuel concentrated near the spark plug to help it burn. This reduces NOx to very low levels, and the engines also employ an additional "NOx trap" to store and destroy the NOx. In this way the required emissions levels can be met, while still giving very good fuel economy.

Unfortunately these engines are very expensive to make, because of the complex technology involved, and also require ultra-low sulphur fuel to give their best. As a result they have had limited sucess in the marketplace, but seem to indicate the likely future for the petrol engine.

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