Talk is cheap

It's easy to promise the world in ads, but it's illegal to make claims you can't prove. We asked eight sellers of fuel-saving devices to justify the claims they made in their advertising.

Several sellers offered us a free example of their device so we could test it ourselves. Their offers were generous, but the testing should come before the advertising. It's not our job to prove their claims. If the products haven't been proven under rigorous scientific tests, then sellers shouldn't be making claims about their effectiveness.

What about testimonials?

Most of the fuel-savers are promoted by testimonials from satisfied users. But that's not the same as objective, third-party lab tests. For example, you can't be sure about the relationship between the importer/manufacturer and the writer. And you don't know how many letters were received from unhappy customers.

The US Federal Trade Commission looked at fuel-saver testimonials: "few customers have the ability or equipment to test for precise changes in gas mileage after installing a gas-saving product. Many variables affect fuel consumption, including traffic, road and weather conditions, and the car's condition."

And then there's the placebo effect. Anyone who's concerned about fuel consumption and fits a fuel-saving device is also likely to back-off on the accelerator too.

We're not trying to knock happy customers. But for these and other reasons, we suggest you ignore testimonials and ask for objective proof before parting with your money.

Does your engine need help?

If you grew up fiddling with carburettors and distributors, you'll be surprised how efficient a new car can be. Modern engines use computers to maximise fuel efficiency while reducing exhaust emissions.

The computer constantly senses the engine's speed, load, the volume of air entering it, and exhaust emissions. It then adjusts the amount of fuel released by the injectors to match the work the engine is doing at that instant.

Adding any additional devices could cause problems. The computer might not be able to make sense of the readings it's getting and makes inefficient adjustments to engine operation. If your car has fuel injection (and most cars built after 1995 do) then you're probably operating about as efficiently as you can. Replace your spark plugs and air filter regularly and see our recommendations.