Mitsubishi Motors Corp. told Japanese regulators that problems with its reporting of fuel efficiency numbers likely affected all vehicle models sold in the country.
Last month, the automaker announced that its employees improperly reduced the resistance of vehicle tires during tests of four "mini-car" models. The resulting numbers effectively improved the vehicles' fuel economy by 5 to 10 percent.
This week, however, Mitsubishi said that an internal probe showed that similar cheating was used on nine models sold in Japan as well as discontinued models in the Japanese market.
Mitsubishi said that fuel economy levels were wrongly increased five times amid concern about competitors' fuel efficiency and that its employees viewed overly ambitious fuel economy targets as "absolute."
The company added that managers failed to properly communicate with those conducting the tests.
Mitsubishi said it is considering "drastic reforms" to ensure similar problems do not happen again. The company is still weighing how to compensate affected owners, and the production and sales of the four mini-cars — including two supplied to Nissan — was suspended last month.
Officials said that Mitsubishi fulfilled testing requirements in its vehicles sold overseas, but U.S. regulators have ordered additional testing of the five Mitsubishi models sold there.
The company late last month established a panel of outside experts to conduct a more thorough investigation of the scandal.