Carmakers Hyundai and Kia face at least a dozen class action lawsuits following an Environmental Protection Agency investigation in which it concluded that the companies overstated the fuel economy estimates of several vehicles.

The EPA found that the car manufacturers overstated the fuel efficiency on 13 different 2011, 2012, and 2013 vehicles by fudging the numbers from one to six miles per gallon depending on the car. For example, although Hyundai claimed the Elantra got 40 mpg on the highway, EPA tests revealed the car only got 38 mpg.

The companies agreed to lower their estimates on roughly 900,000 cars.

In a joint press release, Hyundai and Kia apologized to their customers and expressed regret about what they termed "procedural errors" at their joint testing facility. In addition to changing the estimates on the cars, the companies said they would reimburse consumers for the difference between the estimated and actual mileage.

Consumers who purchased cars with inflated estimates will receive a personalized debit card "that will reimburse them for the difference in the EPA combined fuel economy rating, based on the fuel price in their area and their own actual miles driven," plus 15 percent for their inconvenience, the companies promised. The debit cards can be refreshed as long as consumers own the car.

Despite the carmakers' assurances of restitution, class action suits quickly followed.

In one complaint, plaintiff Brian Reeves claims he bought a 2012 Kia Soul based on the company's fuel estimate that the car got 26 mpg in the city and 34 mpg on the highway, rather than the actual 23 mpg and 28 mpg the car actually achieved. Reeves seeks compensatory and punitive damages for violation of Missouri state law.

A second suit, filed in California federal court, seeks an estimated $775 million in damages for the class of car owners. Plaintiff Nicole Marie Hunter, who purchased a 2012 Hyundai Accent, relied upon the estimated 40 mpg highway driving estimate and would not have purchased the car or paid as much for it had she known its fuel economy was 3 percent lower, or roughly 38.8 highway mpg or less, she claims.

To read the complaint in Hunter v. Hyundai, click here.

Why it matters: "Consumers rely on the window sticker to help make informed choices about the cars they buy," Gina McCarthy, assistant administrator for the EPA's Office of Air and Radiation, said in a press release. "EPA's investigation will help protect consumers and ensure a level playing field among automakers." Not surprisingly, the EPA's investigation also resulted in multiple class action lawsuits, with almost a dozen filed less than one week after the EPA released its findings.