The admission follows last week's revelation that it had falsified fuel economy data for four "minicar" models sold only in Japan.
Regulations changed in 1991 to better reflect stop-start urban driving, but Mitsubishi failed to heed the change.
"We should have switched, but it turns out we didn't," said Ryugo Nakao, executive vice-president.
That meant many more models than the four minicars may have used fuel tests that did not comply with the regulations.
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Koji Yokomaku, another executive, said Mitsubishi had raised fuel economy targets five times in two years while developing the minicars, ending at 29.2 km (18.1 miles) a litre from an initial target of 26.4km a litre.
Tetsuro Aikawa, Mitsubishi Motors' president, said an inquiry led by three external lawyers had been opened: "We don't know the whole picture and we are in the process of trying to determine that. I feel a great responsibility.