As gas prices continue to soar, schemers are ramping up claims that they have just the right device or additive or some other gizmo for better fuel economy.

But many of the claims are misleading or fraudulent, government agencies and the Better Business Bureau contend.

BBB says it has seen these scams before, especially during the gas crises of the 1970s. Typically, the schemes are for gas-saving devices that attach to your engine, special gas caps or fuel additives. However, because some of these devices can cost anywhere from a few hundred dollars to $1,000 or more, you end up spending rather than saving.

In general, many of these products don't produce, said Cathy Milbourn, spokeswoman for the Environmental Protection Agency.

"Cars are very expensive, and you want to make sure what you are using is going to work," Milbourn said.

The Pennsylvania Attorney General's Office recently warned consumers about falling for alleged fuel-saving products.

"There is no magic way to instantly boost your vehicle's fuel efficiency, so be careful before you spend money on additives, devices or other products that promise dramatically better gas mileage," Attorney General Tom Corbett said in issuing the consumer warning. "These gimmicks or gadgets may give you little or no return on your investment, and also may damage your car or void your warranty."

When gas prices go up, so does the volume of advertising for gas-saving products, according to the Federal Trade Commission.

The pitches are enticing to consumers weary of the drain on their budgets. One Web site promises gas savings of more than 40 percent by converting your car to run on a mixture of water and gas. The do-it-yourself manual costs $150 (plus $60 in raw materials), the site says.

"I haven't seen any independent research proving this is a safe, mass-marketable source of fuel for cars," said Dale Dixon, president and chief executive of the Better Business Bureau serving southwest Idaho and eastern Oregon. Dixon said that even if a gas-saving product works, the gains are small, at best.

For example, Dixon said one company was selling a six-ounce bottle of some gas-saving product over the Internet for $40. The company promised a savings of five miles per gallon.

"First, there was no third-party verification of the claim," he said. "And you have to save a lot more than five miles a gallon to make that $40 bottle pay off. The math just doesn't work."

And yet people continue falling victim to these scams.

"I see desperation from folks who are trying to figure out a way to deal with energy costs as high as they are," Dixon said.

The Environmental Protection Agency has evaluated or tested more than 100 devices promoted as saving gas and has not found any product that significantly improves mileage, said Howard Schwartz, director of communications for the Better Business Bureau of Connecticut, which also issued a warning to consumers.

"People are being inundated with these ads that appeal to their frustration about high fuel costs," Schwartz said. "But these products will simply help empty your pockets with questionable technology."

A Washington, D.C., area technical consulting firm, Energy and Environmental Analysis Inc., estimates that 99 percent of aftermarket products do not change fuel economy, according to Schwartz.

Before you buy one of these devices or products, consider these warnings from the FTC and BBB:

- If a company claims that it has an endorsement from government, that's a big fat lie. The EPA has issued reports based on its testing and evaluation of some products. The agency has posted on its Web site a list of approved additives at http://www.epa.gov/oms/consumer/reports.htm. By the way, if a company claims that its product has been evaluated by the EPA, ask for a copy of the report.

- Be leery of customer testimonials. Come on now. How can you really know if the people praising a product are real? As Dixon points out, few people have the ability or the equipment to test for precise changes in gas mileage.

- Be skeptical that a single product can achieve double-digit savings in your fuel economy.

- Check with a mechanic or professional before installing or using a product. Many of them can damage your vehicle or cause substantial increases in exhaust emissions. If you bought a phony gas-saving device, contact your state attorney general or the FTC.

The fact is, many significant gas-saving strategies don't involve you paying a penny. Here's one big one. Generally, for every five miles per hour you drive over 60 miles per hour, it's the same as spending another 30 cents per gallon, Milbourn said.

For additional tips to get better fuel economy, go to http://www.fueleconomy.gov, a Web site jointly run by the U.S. Department of Energy and EPA.

Even as gas prices continue to rise, don't let the pain you're feeling at the pump cause you to fall victim to one of these schemes.

— Listen to Michelle Singletary discuss personal finance every Tuesday on NPR's Day to Day. To hear her reports online, go to http://www.npr.org. Readers can write to her c/o The Washington Post, 1150 15th St., N.W., Washington, D.C. 20071. Her e-mail address is singletarym@washpost.com. Comments and questions are welcome, but due to the volume of mail, personal responses may not be possible.