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The answer is "not much." Be skeptical of the following kinds of advertising claims.
"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 (EPA) 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 vehicle's engine or cause substantial increases in exhaust emissions.
"After installing your product on my car, I got an extra four miles per gallon." 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. For example, one consumer sent a letter to a company praising its "gas-saving" product. At the time the product was installed, however, the consumer also had received a complete engine tune-up -- a fact not mentioned in the letter. The entire increase in gas mileage attributed to the "gas-saving" product may well have been the result of the tune-up alone. But from the ad, other consumers could not have known.
"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 www.epa.gov for information. In some instances, false claims of EPA testing or approval have been made.