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 believed it had been using unapproved methods since 1991 — a period that covers dozens of vehicle introductions and millions of cars and trucks. The company sells a little over 100,000 vehicles in Japan a year, but in the past it sold as many as five times that.
Mitsubishi has been reviewing its tests since the revelations that it had cheated on ones for the mileage ratings of small-engine microcars that 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 on Tuesday. "All I can do is apologize."
Aikawa said that Mitsubishi was still investigating who ordered the cheating, and he suggested that 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.
Aikawa was repeatedly pressed Tuesday to say how many vehicles Mitsubishi now believes were affected. He declined to do so, saying the company was still investigating. But the use of unapproved testing methods appears to have been long prevalent.
"We are examining every model," he said.
Mitsubishi is already a much-diminished brand, especially in its home market, where its market share has shriveled since a scandal in the 2000s. That affair also involved admissions of long-running malfeasance: Officials had been hiding reports of dangerous vehicle defects for decades.
In the latest revelations, Mitsubishi says it cheated Japanese regulators and car buyers 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.
Different countries require different methodologies when testing the fuel economy of vehicles intended for sale in their markets. They may require tests that mimic their own particular driving conditions, for instance more highway driving in the United States, compared with more stop-and-start city driving in Japan.
Mitsubishi said it had begun using the unapproved tests in 1991 and had later incorporated data based on such tests into mathematical simulations that are also used to estimate fuel economy.
In 2007, Japanese regulators notified carmakers that they were adjusting their fuel-economy testing standards. Mitsubishi had new manuals printed, but its development department ignored them, the company said.
In the case of the microcar line, at least, engineers appear to have actively gamed the tests. During development in 2011, Mitsubishi said, it had initially given the vehicle, the eK, a fuel efficiency rating of 26.4 kilometers per liter, or 60 miles to the gallon. But between then and the car's release in 2013, the rating "was repeatedly revised upward in internal company meetings," until it ended up with a published standard fuel economy of 29.2 kilometers per liter.