TOKYO — Mitsubishi Motors admitted on Tuesday that it had used improper methods to test the fuel economy of cars sold in Japan for 25 years, drastically widening the scope of a mileage-doctoring scandal gripping the company.

The automaker said it still did not know exactly how many models had been given exaggerated fuel ratings. But it said it now believed it had been using unapproved methods since 1991 — a period that covers dozens of vehicle introductions and millions of cars and trucks.

Mitsubishi has been reviewing its tests since the revelations that it had cheated on ones for the mileage ratings of small-engine microcars that it sells in Japan and supplies to another Japanese automaker, Nissan, through a joint venture agreement. Nissan engineers discovered the discrepancy last year, the company said.

Last week, Mitsubishi said its cheating on fuel economy tests had affected 620,000 vehicles sold in the Japanese market since 2013.