THE THREE major American automobile companies earned $8.1 billion last year. They owe nearly all of it to the "voluntary restraints" -- that is, mandatory import quotas -- that the Japanese government has now decided to extend through a sixth year. With these import quotas in effect, the prices of American-made cars averaged about $1,000 higher than they otherwise would have. Since the three companies sold 7.8 million cars last year, the figures suggest that in the absence of quotas they would have been running right at the edge of profitability.

The White House has received the Japanese government's announcement in studied silence. While the administration obviously welcomes the extention of the quotas, in deference to its free market principles it prefers not to discuss the subject publicly. It meets all inquiries with slightly defensive observations that trade must be fair as well as free.

But the rise of the yen has already eliminated the one great unfair advantage that the Japanese imports enjoyed. Last summer an undervalued currency gave a subsidy worth perhaps $2,700 to each $10,000 car imported from Japan. Now, with the yen at its present value, that subsidy has disappeared.

There is one powerful reason to keep the quotas in effect another year: they are very helpful to the Reagan administration in its struggle to dissuade Congress from enacting violently protectionist legislation. There was a huge congressional outcry last year when the Japanese raised the quota from 1.85 million cars a year to the present 2.3 million -- one out of every five cars sold in this country.

But the quotas are going to have lasting effects, by no means all of them beneficial to the American automobile manufacturers. Quotas put pressure on the Japanese companies to move upward in the American market toward larger and more expensive models. That upward movement is going to be accelerated by the arrival of small Korean cars built to undercut the Japanese. The effect is to increase foreign competition in the most profitable parts of the American market. While the quotas speed up this process, it won't be reversed when the quotas are eventually lifted.

And when do you suppose the quotas will be lifted? They are immensely costly to people buying cars. They distort competition, and put efficient Japanese manufacturers under restrictions that less efficient manufacturers in, for example, Germany, France and Britain do not have to cope with. The American companies have now had the two most profitable years in their history, and the yen is no longer undervalued. If not now, when will the time be right to drop the quotas?