Mitsubishi Motors Corp said on Wednesday that improper data for mileage calculations might have been used for other models in addition to those it has already disclosed.
The Japanese carmaker last month admitted to overstating the fuel economy readings in four of its mini-vehicle models.
On Wednesday, it said it suspected improper data was used for models among the nine current ones on the market as well as models it no longer sells.
It said it was investigating the matter. It also said it had the financial resources to do with the issue as a whole.
Mitsubishi Motors earlier in the day submitted an additional explanation to Japan's transport ministry after officials found more irregularities with its vehicles following a raid on the company's research facilities last month.
Analysts have estimated that Mitsubishi may have to pay close to $1 billion to compensate Japanese mini-vehicle customers for 'eco-car' tax breaks and extra fuel costs.
The misconduct has revived memories of a scandal more than 15 years ago in which Mitsubishi Motors admitted systematically covering up customer complaints for more than two decades.
Its brand image weakened, it was unable to recover on its own and received a major bailout from other Mitsubishi Group companies in 2004 after which it continued to be plagued by recall and quality issues.