MARK GORDON of Whitestone, Queens, figures that buying six Hyundais over the years classifies him as a loyal customer. “I put 220,000 miles on my last one,” said Mr. Gordon, 58, a manufacturer’s representative for bridal gowns. “They don’t break down.”

Yet Mr. Gordon is so disappointed with the fuel economy of the 2012 Elantra he bought this year, and with the company’s response to his complaints, that he created a protest Web site, my2012HyundaiElantragetslousygasmileage.com.

“This will be the last Hyundai I buy,” he said.

Some other owners have also complained, notably in online forums. A lawsuit filed this month in California Superior Court in Sacramento against Hyundai Motor America claims that multimedia advertising for the 2011-12 Elantra compact sedan trumpeted its 40 miles-per-gallon rating without sufficiently disclosing that this is just one of the car’s economy ratings. The Environmental Protection Agency rates the Elantra at 29 m.p.g. city and 40 highway, with a combined average of 33 m.p.g.

“This advertising campaign was designed to deceive the public,” said Laura Antonini, a lawyer for Consumer Watchdog. the advocacy group that filed the suit. “It was centered around the 40 m.p.g. claim, but this number relates only to the substantially higher highway number.” Ms. Antonini said Hyundai was hiding the fact that the car could not achieve 40 m.p.g. under most driving conditions.

In a statement, Hyundai Motor America said the lawsuit had no merit and that the company’s advertising was “accurate and in full compliance with applicable laws and regulations.”

The Hyundai suit and a recent class-action suit against Honda over the advertised mileage of its Civic Hybrid illustrate consumers’ confusion over federally mandated fuel economy labels and the advertising claims they spawn.

The Hyundai case may hinge on an interpretation of the Federal Trade Commission rules on fuel economy advertising. The agency requires any advertising of mileage to include numbers generated by E.P.A.-approved testing procedures, but it no longer mandates disclosure of both the city and highway numbers, or the combined rating.

The F.T.C. has changed its rules, according to John German, a fuel economy expert who is a senior fellow with the International Council on Clean Transportation. The F.T.C. once required that both the city and highway numbers be given equal emphasis in advertising, but in recent years it has allowed automakers to promote one or the other — usually the highway number, because it is typically higher.

“If someone is just driving around town, they don’t have a prayer of matching the highway number,” Mr. German said. Other driving factors, such as short trips and colder temperatures, will also increase fuel consumption, he said.

At fueleconomy.gov, an Energy Department Web site where drivers can log their mileage, owners of the 2012 Elantra with an automatic transmission reported averaging 29 m.p.g. That is 4 m.p.g. below the combined E.P.A. rating of 33 m.p.g., but within the typical deviation of plus-or-minus 7 m.p.g. for all vehicles tracked by owners on the site, said David L. Greene of the Oak Ridge National Laboratory, which administers the site.

While the Honda and Hyundai lawsuits are about advertising, the dispute underscores the disparity between window-label fuel economy tests and real-world driving. For example, in the federal highway test, cars are run no faster than 60 m.p.h. and with an average speed of 48 m.p.h., significantly slower than most people drive on the highway.

Mr. German said that although the tests themselves might not be realistic, a numerical fudge factor was applied to all cars to make the final label numbers more accurate. In 2008, the government rewrote its testing procedures in an attempt to make the window labels more reflective of real-world results.

Yet Mr. Gordon, who drives his Elantra 30,000 miles a year, says he struggles to get more than 20 m.p.g. “I bought it for the gas mileage,” he said, “but it’s the same mileage as the car I traded in.”