Mitsubishi's fuel economy tests deviated from Japanese standards from 1991 onward. Specifically, the company neglected to update how it conducted coasting tests, in which a car slows to a standstill to measure its air resistance and drag. In 2001, the company found that its tests produced data that deviated from official results by up to 2.3 percent but still refused to update its methodologies.

Ryugo Nakao, the company's vice president, said that regulations had changed in 1991 to reflect the stop-and-go driving style of Japanese city dwellers, but that Mitsubishi continued to use older testing methods. "We should have switched, but it turns out we didn’t," Nakao told reporters, adding that aggressive internal targets may have been triggered the cheating. "Judging by what the investigations have shown so far, it seems there was pressure," he said.

Last week, Mitsubishi said it had manipulated data for four models of cars: the eK Wagon and eK Space, (manufactured and sold by Mitsubishi), and the Dayz and Dayz Roox, (manufactured by Mitsubishi and supplied to Nissan). Production and sales of all four models have now stopped. The company says it will be looking to see what other vehicles were affected by improper testing, and has established a special committee staffed by external experts to investigate the matter.