TOKYO — Mitsubishi Motors admitted Tuesday that it had used improper methods to test the fuel economy of cars sold in Japan for 25 years, drastically widening the scope of a mileage-doctoring scandal gripping the company.
The automaker said it still did not know exactly how many models had been given exaggerated fuel ratings. But it said it now believes it had been using unapproved methods since 1991.
Mitsubishi has been reviewing its tests since the revelations that it had cheated on ones for the mileage ratings of small-engine microcars it sells in Japan and supplies to another Japanese automaker, Nissan, through a joint venture agreement. Nissan engineers discovered the discrepancy last year, the company said.
Mitsubishi gave no indication that it had cheated on tests of cars sold in other markets.
"I'm truly sorry that customers were led to buy vehicles based on incorrect fuel-efficiency ratings," Mitsubishi's president, Tetsuro Aikawa, said at a news conference Tuesday. "All I can do is apologize."
Aikawa said that Mitsubishi is still investigating who ordered the cheating, suggesting the company may not have initially been aware that it was breaking the rules.
Last week, Aikawa said Mitsubishi employees had knowingly given the microcar line an exaggerated fuel rating by manipulating test conditions. The company's share price has dropped by half since then, including a nearly 10 percent decline Tuesday.
Mitsubishi says it cheated by using an unapproved method to measure the effect of deceleration during fuel economy testing. The method, which tends to give a more flattering mileage rating, is approved in the United States but not in Japan.