Be skeptical of the following kinds of advertising claims, according to the Federal Trade Commission:

• This gas-saving product improves fuel economy by 20 percent.

Claims usually tout savings ranging from 12 to 25 percent. However, the Environmental Protection Agency has evaluated or tested more than 100 alleged gas-saving devices and has not found any product that significantly improves gas mileage. In fact, some gas-saving products may damage a car's engine or cause substantial increases in exhaust emissions.

• After installing your product on my car, I got an extra 4 miles (6.4 kilometers) per gallon (3.8 liters).

Many ads feature glowing testimonials by satisfied customers. Yet few consumers have the ability or the 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.

• This gas-saving device is approved by the federal government.

No government agency endorses gas-saving products for cars. The most that can be claimed in advertising is that the EPA has reached certain conclusions about possible gas savings by testing the product or by evaluating the manufacturer's own test data. If the seller claims that its product has been evaluated by the EPA, ask for a copy of the EPA report, or check for information. In some instances, false claims of EPA testing or approval have been made.