Suzuki Motor Corp said on Wednesday it had used emission and fuel efficiency testing methods that were different from Japanese regulations on 16 models now sold in the country, but that proper testing showed no need to amend the data.
Japan's fourth-largest automaker said the improper testing method dated back to 2010 and that around 2.1 million vehicles were affected.
The discrepancy does not affect any Suzuki-badged vehicles overseas, it said in a statement.
Its announcement comes after Japan's transport ministry ordered all domestic automakers to investigate their fuel economy testing methods in the wake of revelations that Mitsubishi Motors Corp used methods that did not comply with regulations.
The automaker specialises in minivehicles, which have engines of up to 660cc and get preferential tax treatment under Japanese law. It commands roughly one-third of the country's minivehicle market.
Fuel economy compliance has come under increased scrutiny since Mitsubishi said last month it falsified fuel economy readings for some of its minivehicle models. It also said it used non-compliant data to calculate mileage for other models.
After Mitsubishi's admission of fuel economy manipulation last month, Japan's transport ministry asked the country's automakers to re-submit fuel economy readings on all their vehicles by Wednesday.
Mitsubishi, which has agreed to sell a one-third stake to Nissan Motor Co in the wake of the scandal, is also expected to give further details later in the day on how many of its vehicles were using non-compliant data to calculate its mileage readings.