Mitsubishi has not responded to a report that it had used improper mileage tests on almost all its vehicles sold in Japan since 1991.
The story in the Japanese newspaper Asahi Shimbun did not identify its sources.
Mitsubishi last month admitted falsifying fuel economy readings for four cars sold in Japan and now says it is investigating whether nine more models may not have been properly tested, including one sold in the US.
It has said tests approved in the United States were used on some vehicles.
In the US higher-speed, highway driving is common, whereas Japanese test standards are set towards fuel intensive city driving which is more prevalent there.