Mitsubishi Motors has admitted using improper fuel tests since 1991.
The admission follows last week’s revelation that it had falsified fuel economy data for more than 600,000 vehicles sold in Japan.
Tetsuro Aikawa, its president, said an investigation was continuing, suggesting that more irregularities might be found.
So much remained unknown that it was uncertain how the company would react, he said.
It was not clear how many cars were affected.
Shares fell a further 10% in Tokyo on Tuesday, bringing the slide since the scandal erupted to almost 50%.
Mr Aikawa said he did not know why employees resorted to rigging fuel economy tests to make mileage figures seem better.
Mitsubishi had repeatedly promised to come clean after a huge scandal 15 years ago that involved a systematic cover-up of vehicle defects.
The inaccurate mileage tests involved 157,000 of its eK wagon and eK Space mini cars, and 468,000 Dayz and Dayz Roox vehicles produced for Nissan. All were sold in Japan only.
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