Suzuki Motor Corp. said Tuesday fuel economy tests that failed to meet regulations affected a total of 2.14 million units across 26 models.
But it added, the models were retested and found to have better performance than their catalog figures.
Speaking at a news conference after submitting a report to the transport ministry, Chairman Osamu Suzuki apologized and expressed his intention to stay on and improve standards.
"Is is the basic responsibility of all employees to abide by the law, so we will make sure that they do," Suzuki said.
The automaker denied that specific individuals were involved in ordering the use of unapproved testing methods for years. It further denied that in-house manuals told employees to use the tests.
The Ministry of Land, Infrastructure and Transport had urged Suzuki Motor to report back with more details by Tuesday.
Domestic automakers were asked to check for irregularities following revelations that Mitsubishi Motors Corp. inflated the fuel economy of some of its minivehicles.
A car's fuel economy determines whether it qualifies for tax breaks under a government incentives program for eco-friendly cars.
Suzuki Motor admitted this month that it performed indoor tests to acquire the resistance data that is needed to gauge fuel efficiency. National regulations require automakers to conduct "coasting" tests on the road.
Suzuki said its testing facility, located on a hill near the sea in Shizuoka Prefecture, tends to be affected by wind and this made it difficult to obtain consistent data.