Things are getting worse for Mitsubishi Motors. In less than a week after admitting that it had falsified mileage tests, the automaker revealed Tuesday that it has been engaging in this practice for the past 25 years - drastically widening the scope of the mileage-doctoring scandal the company finds itself mired in.

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. The automaker initially said this practice went back to 2002.

Mitsubishi has been reviewing its tests since it revealed last week that it had cheated on tests for the mileage ratings of so-called "minicars" that it sells in Japan and supplies to another Japanese automaker, Nissan, through a joint venture agreement. The problem surfaced after Nissan brought it to Mitsubishi's attention last year, the company said.

Specifically, the automaker said it compiled data for fuel economy tests using U.S. standards, where higher-speed, highway driving is common, as opposed to Japanese standards, where more prevalent city driving commonly consumes more fuel. Mitsubishi Motors said the U.S. testing method might have been used as it is shorter and would save time and was likely the result of  aggressive internal targets which could have possibly put pressure on employees to overstate the fuel economy of its vehicles.

All of this means that even though the confirmed tests only involved 157,000 of Mitsubishi's eK wagon and eK Space light passenger cars, and 468,000 Dayz and Dayz Roox vehicles produced for Nissan Motor Co., other vehicles may now have overstated fuel economy values as well.

Even with all of this new knowledge, Mitsubishi President Tetsuro Aikawa said that the investigation is still ongoing, which means its still likely that more irregularities may be found.

"We don't know the whole picture and we are in the process of trying to determine that," he said. "I feel a great responsibility."

Mileage fraud is a violation of Japan's fuel efficiency law for autos because buyers are eligible for tax breaks if a vehicle model delivers good mileage. Possible penalties are still unclear due to the uncertainties over the investigation's outcome. However, even if legal penalties are unclear, financial ones are as clear as day. After revealing that it had cheated on the tests, Mitsubishi lost about $3.9 billion - or half of its market value.

This whole situation revives memories of a scandal more than 15 years ago in which Mitsubishi Motors admitted to systematically covering up customer complaints for more than two decades, bringing the company close to collapse. The brand was tarnished even further in 2002 when a woman who was walking down a sidewalk was struck and killed after a tire rolled off a nearby Mitsubishi truck.

In the past, other Mitsubishi firms bailed the company out of trouble, however there may be no such luck this time around. Senior officials at other firms say that if needed, it would be difficult for them to help the car maker this time, as they have their own financial problems to deal with thanks to this debacle, as well as calls to put shareholder returns above ties with the former Mitsubishi business empire.