IF YOU EVER prepaid a consumer loan, the odds are your debt was calculated by a shorthand method called "rule of 78's." These days it works to increase the finance charges collected by the banks and small loan companies that use it.

In introducing legislation recently to outlaw it, Sen. Paul E. Tsongas cited a family in Indiana who borrowed $18,000 for 15 years at 18 percent interest. After four years of monthly payments, they decided to pay off the loan -- only to discover that the finance company said they now owed $19,933.The number was right out of the rule of 78's book and was about $3,000 higher than an accurate calculation of the 18 percent interest. The number was also legal: the truth-in-lending act permits use of the rule of 78's as long as the loan agreement says it will be used for prepayments.

The problem is that the calculations were made in the 1930s. and worked resonably well as long as the length of the loan was short -- two or three years -- and interest rates were low. Although the rule always produces figures that slightly favor lenders, the differences didn't used to be enough to worry about.

Now that consumer loans are sometimes made for long periods of time and interest rates are high, the difference between the old tables and actual interest calculations can be huge. Some experts think the tables produce as much as $100 million a year more for lenders than accurate interest calculations would.

Almost no one defends the rule of 78's on its merits anymore. The invention of electronic calculators has made it easy for lenders to do that complex arithmetic, thus eliminating the need for inexact tables. But the government's banking agencies asked a senate subcommittee the other day for time -- six months or so -- to study the proposed legislation.

Their problem, and the problem of many lending institutions, is that state usury laws are forcing lenders to rely more and more on this and similar prepayment gimmicks -- like the points charged on mortgage loans -- for their profits. Many of those laws put a cap on the interest rate a lender can charge that is unrealistically low in terms of the interest the lender must pay on the money it has borrowed.

This is an appropriate time for both Congress and the state legislatures to look at this situation. As the economy slows down, more creditors are going to get in trouble on consumer loans. As they do, one instinctive reaction will be to refinance. And as debts are refinanced, the toll taken by the rule of 78's grows larger. While there may not be any easy way to abolish this and the other financial gimmicks, the need to do so is clear.