Japanese automaker Mitsubishi Motors said the company has been using an improper method to determine the fuel efficiency of its vehicles for 25 years.
Tuesday's admission by President Tetsuro Aikawa in Tokyo follows last week's revelation that Mitsubishi had falsified fuel-efficiency data on 625,000 vehicles.
Aikawa said the company's latest troubles occurred because it failed to update its testing system to conform to domestic standards.
The issue was revealed as part of an investigation into scandal over false fuel-efficiency data.
The first false data reports dated back to 2013, involving Mitsubishi's domestic eK Wagon and eK Space Custom light passenger cars, as well as the Dayz Roox vehicles it produces for rival carmaker Nissan.
The problem was uncovered after Nissan noticed inconsistencies in its data.
Mitsubishi has stopped selling the vehicles while it conducts the probe.
The world's sixth-largest automaker has lost more than half of its market value since news of the fuel-rigging scandal broke.
The company is no stranger to scandal. It nearly went out of business after admitting back in 2000 that it covered up major safety defects for several decades.
Mitsubishi is the second carmaker accused of falsifying environmental data in the past 12 months.
Germany's Volkswagen admitted late last year that it installed software on millions of its vehicles that activated bogus emissions controls to deceive testing officials.