Touting “best-in-class” fuel economy and eye-popping Environmental Protection Agency ratings once was the norm for U.S. auto makers seeking to draw the attention of car buyers facing ever- increasing gasoline price hikes.

The marketing ploy worked for a while, with Hyundai at one point boasting 40 mpg (5.9 L/100 km) ratings on more vehicles than any other brand. But a crack in the strategy slowly widened as astute consumers began noticing their fuel economy wasn’t as advertised.

Hyundai and sister brand Kia were the first to be called out, prompting the two to apologize for their inflated-mileage claims and reimburse thousands of customers for their higher-than-expected fuel costs.

Ford was next to feel the sting when it unveiled its new hybrids, the C-Max and Fusion, claiming the models achieved a combined 47 mpg (5 L/100 km) city/highway. The influential buying guide Consumer Reports was the first to point fingers at Ford, when during internal testing, it was unable to achieve anywhere near the advertised fuel economy.

Although the auto maker was quick to point to differing driving styles, weather conditions and other outside factors that affect fuel economy, it was too late. Lawsuits quickly piled up, including a class-action suit filed against Ford by the California-based McCuneWright law firm.

That suit, a copy of which was obtained by WardsAuto, contends false advertising led to record sales of the C-Max in the car’s first two months in the market.

“Ford engaged in widespread misleading and deceptive advertisements regarding the real-world gas mileage of the C-Max and Fusion hybrid by promoting inflated gas-mileage numbers when the vehicles, in fact, fall substantially short of attaining in real-world, normal use,” the suit says.

The suit was filed on behalf of C-Max owner Richard Pitkin and is open to “all others similarly situated.” It asks for full reimbursement of the vehicle purchase price as well as unspecified damages.

Ford argues it strictly adheres to EPA testing in formulating its fuel-economy ratings and that many of its C-Max and Fusion hybrid customers have surpassed 47 mpg.

The auto maker is working with the EPA to determine if its test protocols, which were developed in the 1970s, could be changed to better reflect the fuel-economy benefits of new technologies such as hybrid-electric vehicles.

Ford also emphasizes new-vehicle window stickers are meant as estimates of the fuel-economy customers can expect to achieve, and results may vary.

“The EPA ratings are a very important common measuring stick, so that consumers are easily able to compare vehicles on an “apples-to-apples” basis,” Ford says in a statement provided to WardsAuto.

The EPA declines to comment on the matter, but a source close to the situation tells WardsAuto that unlike Hyundai, Ford is not being investigated by the agency and that it complied with federal-testing rules.

Ford, Hyundai and Kia likely are not the last auto makers to be affected by inaccurate fuel-economy ratings, and it’s likely to cause other brands to rethink their marketing strategies.

The real problem may lie in the EPA’s test cycle, which does not include real-world testing in an effort to mitigate variable outside factors, including weather and road conditions, from skewing results.