Misubishi Motors Corporation (MMC) admitted that it has improperly tested the fuel economy of its cars for the past 25 years, in a scandal that has already halved its market value in a week.

The automaker created a panel of former prosecutors to investigate the manipulation of fuel economy data that goes back as far as 1991, a quarter of a century ago.

"MMC expresses its most sincere apologies to all of our customers, shareholders, and stakeholders for any inconvenience or concern caused by this occasion," MMC says in a news release.

The revelation came from a press release titled "Improper conduct in fuel consumption testing on products manufactured by Mitsubishi Motors Corporation (MMC)," wherein the company found that the fuel consumption tests show improper data during the certification process of mini-vehicles. The findings reveal that they gave out better fuel consumption results than the actual rates.

The company also revealed that it had used a different method of fuel testing from what is imposed by Japanese law.

"Customers bought our cars based on incorrect fuel-economy data," President Tetsuro Aikawa said in a press conference on April 26.

"I can't help but apologize," he added.

Mitsibushi, Japan's sixth largest automaker, suffered a plunge since it admitted the scandal, decreasing the automaker's market value to 427 billion yen ($3.85 billion). Aikawa said the company hasn't decided on how it will compensate customers but added that he was not aware of the testing discrepancy.

The mini-cars affected include four models which were sold only in Japan. In 1991, regulations were changed but the company failed to join the modification. This means that many other models may have used faulty fuel tests since the company did not comply with the new regulations.

"We should have switched, but it turns out we didn't," said Ryugo Nakao, executive vice-president.

MMC said it is still investigating the total number of vehicles involved in the scandal. In the report released on April 27, the automaker said that a total of about 625,000 units of the four mini-car models were affected.

The U.S. Environmental Protection Agency said it has ordered MMC to provide additional data and conduct new tests of vehicles in the United States after the company's admission of the fuel economy tests.

Photo: Steven Johnson | Flickr