Apparent plans by Japan to continue restraints on auto exports to the United States but to allow a 25 percent rise to about 2.3 million vehicles a year are being depicted here as a gesture of conciliation to help defuse the two countries' trade tensions.

Japanese officials and auto executives argue that the original rationale for the four-year-old quotas, financial difficulties of the U.S. auto industry, has long ago disappeared. Only political justification remains, they say.

A recent government poll of the nine Japanese auto manufacturers' export plans if restraints were eliminated reportedly found that exports could rise to 2.7 million cars a year or more, or about 400,000 more than Japan reportedly is contemplating.

Officials here said today that no firm decision on a final number has been reached. Some reports said an announcement will be made following a Japanese Cabinet meeting Friday, but other officials said it may take longer to reach a decision.

A Foreign Ministry official contested a statement yesterday by White House spokesman Larry Speakes that exporting 2.3 million autos would take the Japanese industry to full productive capacity. Japan can go much higher, he said.

Japan's decision is coming as it is embroiled in a bitter trade dispute with the United States, which has a $35 billion yearly trade deficit with Japan and is pressuring Tokyo to take steps to reduce the deficit. The two countries are discussing trade in telecommunications, electronics, wood products and medical products.

One month ago, citing "the wisdom of maintaining fair and free trade," President Reagan announced that he would not press Japan to renew the current quotas, which will expire Sunday. He asked Japan to respond with market-opening steps of its own. However, many Japanese felt the words were insincere, an easy means of winning praise as a free trader, when through a variety of informal channels the United States had been indiciating that it would not welcome the surge of car imports expected if quotas were abandoned.

There is still some confusion in the Japanese government. "I really would like to know where American opinion lies," one government official said.

One official here suggested yesterday that Japan faces a no-win situation, because Washington is alternately "chasing after two rabbits," free competition and prevention of an import surge.

The controls that are about to expire allow 1.85 million Japanese autos to be shipped to the United States each year. In recent weeks, the Ministry of International Trade and Industry, the auto industry and politicians have been debating the next step.

All sides in Japan are believed to have begun with the assumption that to preserve relations with the United States, free trade in automobiles was for now impossible. However, there were major differences on what should be the new control level and administrative means.

Facing a stagnant market at home, manufacturers pushed for permission to increase U.S. sales. However, there was a division between the major companies, which are doing well under the quotas, and minor ones, which have been largely barred from U.S. sales.

Industry giants Toyota and Nissan, which between them control more than half the current quotas, were said to be willing to live with comparatively small increases. But small companies protested loudly.

Isuzu, for instance, has invested heavily in a new plant to supply General Motors with almost 200,000 subcompacts a year for sale as GM models and has been especially anxious to win freer access to the U.S. market.