NEW YORK (CNNMoney.com) - It costs less than $40 and, according to its Web site marketing, it will raise your vehicle's fuel economy by 30 percent or more. Customers swear by it in testimonials.

Whatever it is, your best bet is to keep your credit card in your wallet.

Whenever gas prices spike, such product pitches spike as well, said Claudia Bourne Farrell, a spokeswoman for the Federal Trade Commission, the government agency responsible for investigating bogus marketing claims.

"More people than you'd probably think" get taken in by bogus claims of big fuel savings with simple devices, Bourne Farrell said.

This time around is no different. And all these come-ons should set off that "yeah, right" alarm in your head.

For example, car companies develop gas-electric hybrid systems costing thousands of dollars to achieve fuel-efficiency improvements...but you just got an e-mail saying you can do it just by installing a tiny fan inside a hose. ("Yeah, right.")

Gasoline companies spend millions of dollars on research and development...but they somehow missed this enzyme that could provide a 30 percent increase in fuel economy. ("Yeah, right.")

Alas, the age of simple solutions is over, said John Millett, a spokesman for the Environmental Protection Agency. "If it worked that well, wouldn't your engine be that way, wouldn't the gas be that way, already?" he said.

In decades past, said Millett, a few novelty devices the EPA tested actually did provide some increase in fuel mileage. But often, those small increases came at the expense of damage to the engine or an increase in harmful exhaust emissions.

Those technologies that did provide a benefit - for example, a device that shut off the air conditioner compressor when the car was idling - have long since been incorporated into virtually every new car built

Many companies will go to great lengths to make their products seem legitimate. Many alleged fuel-saving compounds are sold with the message that they are "registered with the EPA" or "approved by the EPA."

That only means that the EPA has accepted paperwork filed by the manufacturer and, based on that paperwork, decided that the fuel additive was no more harmful to the environment than gasoline. It doesn't mean that the EPA found it to be effective. In many cases, the seller may not even have done that much and may simply be lying.

The EPA tests very few new devices that claim to save fuel. The EPA will only test a device if the manufacturer specifically requests it or if the FTC requests the test to back up an enforcement action.

Copies of the EPA's test results are posted on the agency's Web site. Overall, those results support the idea that consumers should approach alleged fuel-saving devices with, at the very least, a great deal of caution, said Millet. None of the devices tested produced appreciable fuel economy gains, he said.

Some concepts -- such as magnet-based devices and devices that go into the vehicle's air intake system -- have already been proven ineffective and would no longer even warrant testing, Millett said.

Just last year, the FTC got an injunction against one company that used e-mail marketing to sell a magnetic device that, the company said, would improve mileage by 27 percent. (The manufacturer in that case, while accepting the injunction, denied any wrongdoing in court documents.)

Similar cases are in the works, Bourne Farrell Said.

Related:

Take it slow and save big on gas

How to shop for a fuel-efficient car

GM exec: Driving is dead

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