AMERICANS SPEND freely, borrow heavily and, notoriously save little. That contributes to the poor performance of the economy. The president-elect is more or less committed to a kind of broad tax cut that would have little effect on savings. Both he and Congress need to think again about that commitment, and move to more sharply focused tax changes to encourage a higher savings rate.

Out of every $20 of income, after taxes, Americans currently save less than $1. That's less than one-fourth the Japanese savings rate, and about one-third the German rate. The Japanese and Germans are well known to be worrywarts, and, you would have to agree, there's a certain historical justification for it. But, even by past American standards, the present rate in this country is recklessly low. One reason, surely, is a decade of high inflation. Another reason, equally surely, is the irrational treatment of savings and interest by the income tax laws.

Interest earned on savings is taxable, but interest paid out on loans and mortgages is deductible. That discourages saving, and encourages borrowing. Fifteen years ago, when the mortgage rate was 6 percent and the prime rate under 5 percent, the amounts of money were hardly big enough to make much difference. Now, with mortgage rates at 15 percent and the prime at 21 percent, interest and the taxation of it are becoming a large consideration in most families' finances.

Are today's interest rates high? That depends on your tax status. suppose that you are fortunate enough to have an income putting you in the 54 percent bracket. Suppose further that you have done all your Christmas shopping on a credit card charging you 18 percent a year. Since that interest is a deduction from your taxable income, the federal government is actually paying 54 percent of it. That makes the after-tax cost to you only 8.3 percent. Since that's less than the present inflation rate, the real cost -- adjusted for inflation -- of this interest charges to you is negative. It actually pays you to borrow. Is it really surprising that consumer borrowing keeps increasing?

The solution is simple, and the Reagan administration will have an opportunity to write it into the coming tax bill. Why not impose a limit on the deductions for interest paid out, and allow a limited tax exemption for interest earned on savings and investments? It would not only be good economic policy, helping to slow down inflation. It would also be good social policy, discouraging the superheated real estate speculation that is rapidly pushing even modest houses beyond the reach of most families.