Washington — It is starting to cost consumers less to buy on credit. On Jan. 1, for example, Eastern Airlines chopped the interest rate on its credit cards from 18 to 11.88 percent. Meanwhile, late last month the Bank of America dropped the fee on four-year car loans to 15.25 percent. Two months ago the loans were priced at 17.5 percent.
Although a wide variety of lending institutions have lowered their fees recently, major additional changes in consumer interest rates are not expected. And the interest rate levels anticipated for 1983 as a whole will leave many shoppers extremely cautious about using credit, economists say.
''Consumer loan rates may come down a little more - as much as one percentage point,'' says a government economist who asked not to be named.
''Most consumer loan rates could go down one to two percentage points'' during 1983, adds Irwin L. Kellner, chief economist at Manufacturers Hanover Trust Company.
Even a further drop of two full percentage points would leave average fees for auto loans at 14 percent or more at year's end, and charges for other major consumer purchases at 16 percent or higher. Little change is expected in bank credit card rates, which averaged 18.75 percent at the end of 1982.
''Something in the neighborhood of 12 percent in consumer loan rates appears to be a threshold,'' notes David Munro, an economist at General Motors Corporation. ''It is fairly important to the prospects for retail sales, including autos, that loan rates not go over the neighborhood of 12 percent.'' That is one reason GM, along with the other major car companies, is offering 11. 9 percent loans to boost car sales.
''To the extent consumer financing rates stay high, it is an obstacle to spending, particularly on durables and cars,'' adds Sandra Shaber, senior economist with Chase Econometrics, a forecasting firm.
Consumers have benefited much less than corporations from recent declines in interest rates. Since July the prime rate, a benchmark for corporate interest fees, has fallen from 16.5 to 11.5 percent, or five full percentage points.
But the charge for an average one-year consumer loan dropped only 1.04 percentage points in the fourth quarter of 1982, from 19.37 percent in the third quarter down to 18.33, according to the most recent Federal Reserve Board data.
Charges on loans to individuals are always slower to change than are lending fees for corporations. ''Consumer loans always come down with a lag'' from corporate rates, notes Rosanne M. Cahn, a vice-president and economist at Goldman, Sachs & Co. One reason is that most consumer loans carry a fixed interest rate over a number of years, while many corporate loans float with market rates.
But bank officials and economists say there are a number of reasons lenders are now likely to be especially cautious about lowering loan rates.
''In the banking sector as a whole, there is a tremendous amount of institutional change going on,'' says GM economist Munro. ''That is obviously leading to uncertainty about when and by how much different rates should move.''
One example of the changes in banking is the introduction Wednesday of Super NOW (negotiable order of withdrawal) checking accounts, on which unlimited interest can be offered. That raises the cost of the money banks lend, putting upward pressure on the fees they seek for loans.
The large losses some banks recently suffered on consumer lending operations may also delay rate declines. ''Banks are just now becoming profitable in their consumer lending activities,'' says Mr. Kellner at Manufacturers Hanover. When the cost of funds moved up sharply in 1980 and '81, consumer loan rates did not keep pace, he explains.
He also notes interest rates have changed so dramatically in recent months, banks want to make sure their cost of funds to lend will be fairly stable before they reset loan charges.
Even the relatively modest changes expected in consumer interest rates will help make purchases more affordable and provide some additional momentum for the economy, says Ms. Cahn at Goldman, Sachs.
For example, if loans for major consumer purchases, other than cars, drop to the 16 to 17 percent level from about 18 percent now, payments as a percentage of household disposable income will be 5 percent, the lowest level since 1972, according to Goldman, Sachs data.
And if car loans, excluding special promotions, drop to the 14 to 15 percent area, loan payments as a share of disposable household income would drop from 11 .6 percent currently to a more normal 10.9 percent. ''While the $15 to $16 saving per month does not seem large, it reduces the total cost of the auto plus financing by over $700, or 6 percent,'' the Goldman, Sachs economist notes.