Automaker Mitsubishi recently admitted to fudging fuel mileage data for more than 600,000 vehicles sold in Japan, leading to an ongoing probe by U.S. regulators. Those investigators may now have a lot more paperwork to sift through, after Mitsubishi’s latest revelation.

The New York Times reports that an internal investigation by Mitsubishi found that the carmaker has used unapproved fuel testing methods for the vehicles sold in Japan for the last 25 years.

The unapproved method measured 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 tests for fuel economy. For example, the U.S. requires more highway driving in tests, while Japan requires more stop-and-start city driving, the Times reports.

For now, there’s no evidence that Mitsubishi cars in the U.S. include the same fuel inaccuracies as the 625,000 small cars sold in Japan, which included some vehicles manufactured for Nissan.

According to Mitsubishi tire pressure data was manipulated to make mileage appear 5% to 10% better than it actually was for 157,000 Mitsubishi eK wagon and eK Space light passenger cars, and 468,000 Dayz and Dayz Roox vehicles produced for Nissan Motor Co.

Japanese regulators announced in 2007 that they were chaining fuel-economy test standards. Mitsubishi says it had new manuals printed, but that development ignored them, the Times reports.

While it’s unclear just how many vehicles and models might be affected by the expanded falsified fuel ratings, it could reach into the millions.

“I’m truly sorry that customers were led to buy vehicles based on incorrect fuel-efficiency ratings,” Mitsubishi’s president and CEO Tetsuro Aikawa said at a news conference on Tuesday. “All I can do is apologize.”

Aikawa says the company is unaware who ordered the cheating, but that the investigation is ongoing.

In fact, the Associated Press reports that the carmaker has arranged for a panel of three lawyers to continue investigating the issue from on outsider’s point of view. The group is expected to have a report completed within three months.

Mitsubishi Says It Cheated on Fuel Tests for Decades [New York Times]
Mitsubishi Motors says false mileage tests done since 1991 [The Associated Press]

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