With the accelerated increase in fuel prices the interest in gadgets and ways to save gas are also increasing. Just a quick investigation reveals that this is nothing new. The Environmental Protection Agency in the US, www.epa.gov , publishes a long list of Gas Saving Devices and Products that they have tested. The earliest of these dates back to 1970.

Amongst the EPA tested gas saving devices and products most did not live up to their promises. A few have saved small but measurable amounts of gas. Unfortunately this list is not really up to date. At the time of writing the most recent product tested was in 2005, a product called Super FUELMAX, and this product has since apparently been withdrawn from the US market.
The products tested fall into various categories, the most prominent being:

Ignition Enhancement Devices – Replace original parts of the ignition system.

Fuel Line heaters, coolers, magnets – These devices heat or cool the fuel before it enter the carburettor or claim to change the molecular structure of gasoline.

Air Bleed, Vapour Bleed or Liquid Injection Devices – These devices introduce air, vapour or liquid into the carburettor.

Fuel Mixture Enhancers – Trying to enhance the mixing or vaporization of the air/fuel mixture.

Fuel and Oil Additives – These materials are added to the gas tank or poured into the crank case.

Miscellaneous – Various additions to change the amount of power used by the system or to modify driving behaviour.

This site can be useful for gas saving devices and products that appear on the list but none of the products that are currently heavily advertised on the internet, such as the water for gas type products, appear there. See Water4Gas here.