The lawsuit is the second of its type filed recently against an automaker and highlights the aggressive use of mileage numbers by manufacturers in advertising their vehicles.
The litigation also underscores growing consumer frustration over mileage claims as the auto industry has shifted its marketing focus from performance attributes such as horsepower and speed to fuel economy, now among the top features consumers look for when buying a new car.
"This is becoming a customer satisfaction issue," said Aaron Bragman, an analyst with IHS Automotive. "If you sell them on an attribute they don't get, it can come back and bite you."
The latest suit contends that Hyundai pitches its "40 Mile Per Gallon Elantra" without a government-mandated disclosure that the estimate is for highway driving only and that other measures are much lower.
It was filed in Sacramento on behalf of resident Louis Bird, who bought an Elantra last year and kept a mileage log. Bird said most of his driving is on open highways but that he barely gets 29 miles per gallon.
"We are hoping that other car makers will take notice and realize that if you do it wrong, you will have to pay the price," said Jamie Court, executive director of Santa Monica-based Consumer Watchdog, which jointly filed the suit.
The lawsuit seeks class-action status and wants to stop Hyundai from using gas mileage numbers in its advertising of the Elantra without government-mandated disclosures. It asks for unspecified damages on behalf of California residents who bought or leased 2011 and 2012 Elantras.
Hyundai defended the estimate, noting that it was certified by the Environmental Protection Agency, which publishes fuel economy ratings for all vehicles sold in the U.S.
"Hyundai Motor America believes this case has no merit, as our advertising is accurate and in full compliance with applicable laws and regulations," the automaker said.