At the center of a debate over mileage claims: the 2012 Hyundai Elantra.

Hyundai Motor America has emerged as the U.S. auto market’s fuel economy leader. In fact, a new report from TrueCar.com, an automotive research firm, indicates Hyundai was instrumental in boosting the mileage of the typical vehicle sold in June to 23.0 mpg in June 2012 compared to 21.6 mpg in June 2011.

Hyundai’s own average mileage was 27.7 mpg in June, the highest in the industry.  The maker notably claims it now has five models delivering better than 40 miles per gallon on the highway based on EPA-approved numbers.

But has Hyundai actually rigged the deck?  That’s what a new lawsuit is alleging.  The maker has been tagged in a lawsuit by an organization calling itself Consumer Watchdog filed by Cuneo Gilbert and LaDuca LLP of Santa Monica. They claim the Korean carmaker has used misleading fuel economy claims in its advertising.

Referring to advertising for Hyundai’s compact Elantra line, Laura Antonini, a staff attorney for Consumer Watchdog, argued that, “Hyundai used the ’40 MPG’ figure in a deceptive manner in order to differentiate the Elantra from similar vehicles, an especially egregious tactic during a time when consumers are looking for relief from continually rising gasoline prices.”

The carmaker categorically denied the charges, spokesman Miles Johnson asserting that, “Hyundai Motor America believes this case has no merit, as our advertising is accurate and in full compliance with applicable laws and regulations. In fact, we’ve reviewed our ads and think Consumer Watchdog and their client are dead wrong.”

There’s a growing debate over both the way fuel economy is calculated and then how the officially approved numbers are advertised.  Honda recently worked out a settlement in a class action lawsuit concerning the mileage actually delivered by early versions of the Civic Hybrid. But the maker has also beaten back some well-publicized individual cases filed by Civic Hybrid owners – including one taken to small claims court in California.  The owner won an initial judgment of nearly $10,000 but later lost when Honda appealed.

Hyundai has been the target of growing criticism by Consumer Watchdog.   The group’s lawsuit claims the maker touted “The 40 Mile Per Gallon Elantra” in high-impact television, Internet, and print advertisements without government-required disclosures that those mileage estimates were for highway driving only and that city driving mileage estimates were much lower.

The omitted disclosures would have informed consumers that the car does not attain 40 MPG under most driving conditions, the group claimed, adding that the allegedly illegal advertisements caused tens of thousands of California drivers to purchase or lease 2011 and 2012 Elantras and consequently incur unexpected fuel costs, the lawsuit said.

“I feel like Hyundai took advantage of me. Hyundai’s advertisements about the ’40 MPG’ gas mileage of the Elantra instantly caught my attention. I bought the car thinking I would be seeing major savings at the pump and getting over 500 miles per tank, but Hyundai fooled me,” said Louis Bird of Sacramento, California, a 2011 Elantra owner who is representing other consumers in the class-action lawsuit.

The lawsuit seeks to stop Hyundai from using gas mileage numbers in its advertising of the Elantra without government-mandated disclosures and asks for damages on behalf of California residents who purchased or leased 2011 and 2012 Elantras.

Hyundai, however, insists it has clearly indicated in press releases and other material it distributed about the Elantra the vehicle got 40 mpg in highway driving on tests certified by the U.S. Environmental Protection Agency.  It intends to defend itself if the lawsuit ultimately goes to court.

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