The cost of financing a car or taking out a personal loan can vary by as much as 50 percent depending on where you live in the greater Washington area, according to a survey of banks conducted by The Washington Post during the last two weeks in June.

Fifty-one percent of the banks surveyed in the District, suburban Maryland and in Virginia as far as Richmond answered the questionnaire. They reported that their annual interest rates on automobile and consumer loans ranged from 12 to 18 percent. The highest were in Maryland, due to the state usury law, and the lowest were in Virginia.

Based on the responses, the median rate charged for the purchase of both new and used cars -- including loans of all maturities -- was 13.98 percent. (The median means there were as many rates reported above as below that figure.) Where the lenders specified 36-month loans for new cars, the median interest rate was 13.69 percent; on 48-month loan, 13.51 percent. On used cars, the rate was 14.66 percent.

The median rate for auto loans in the District was 14 percent, 14.5 percent in Maryland and 13.84 percent in Virginia. The lowest rate recorded in the entire area -- 11 1/2 percent with a 25 percent down payment -- was offered by the National Capital Bank of Washington on Pennsylvania Avenue SE.

Chairman George A. Didden Jr. said his bank's rate has long been the lowest in town. The loan is made for a maximum of 36 months on 75 percent of the actual discounted invoice price, not including title, tax or tag fees and a $5 credit investigation fee.

Central National Bank of Maryland, located in Silver Spring also charges 11 1/2 percent on auto loans of up to 48 months, provided payments are automatically debited to the owner's checking account. When the payments are not made automatically, the rate goes up to 12 percent.

The next-lowest rate reported as 11.94 percent by the State Bank of Prince William County in Dumfries, Va.

In Maryland, loans at 12 percent were listed by Bank of Damascus and Peoples Security Bank of Maryland in Suitland. Also in under 13 percent were Farmers & Merchants National Bank of Hamilton, Va., which offers 36-month loans on new cars for the equivalent of 12.83 percent; Round Hill National Bank of Round Hill, Va. (12.68), and Guaranty Bank & Trust Co. of Merrifield, Va. (12.83). (Actual annual percentage rates may differ slightly because required down payments, loan maturities and other special conditions were not always specified by the respondents.)

Figuring loan rates in Maryland is complicated by state usury laws that allowed lenders to charge 18 percent on loans up to $3,500 and 12 percent on amounts above that. On many used cars, therefore, the loan rate was effectively 18 percent.

As for new cars -- assuming $6,000 as the price of the average car, minus a 25 percent down payment -- one calculates the average loan at $4,500, with an annual percentage rate of 16.6 percent. The vast majority of the Maryland banks surveyed employed this rate structure.

However, starting July 1 the dollar limit will be lifted, enabling banks to charge up to 18 percent on any unsecured loan. With interest rates and car sales down, lenders are not likely to take full advantage of this. However, car financing may rise somewhat. Union Trust Bank, for example, has announced its auto loans will be between 14 percent and 16 percent, "subject to prevailing market conditions."

As for consumer or unsecured personal loans, the survey showed the rates ranging from 11 1/2 percent to 18 percent. The median interest rate for the greater Washington area was 14.68 percent during the last two weeks in June. Again those Virginia banks that responded had the lowest median, 13.98 whereas the District median was 15 percent.

Central National Bank of Maryland offered installment loans of over $5,000 for 36 months at 11 1/2 percent, provided payments were automatically debited to the borrower's checking account; otherwise the rate was 12 percent. Most other Maryland banks surveyed offered loans of more than $3,500 at 12 percent in June. (Undoubtedly most will increase their rates as of July 1 when the dollar limitation is lifted.)

Farmers & Merchants in Virginia reported unrestricted 12 percent consumer loans. Guaranty Bank & Trust (12.88) was the only other institution below 13 percent.

Those charging under 14 percent where: First Virginia Bank in Falls Church (13) Bank of Virginia in Vienna (13.98), The McLean Bank (13.75), Middleburg National Bank (13.78), State Bank of Prince William County (13.94), Continental Bank & Trust in Springfield (13.98) and Burke & Herbert Bank & Trust in Alexandria (13.86).

In the District, three out of 10 banks answering the questionnaire reported installment loans of 14 percent. They were the Madison National Bank, Industrial Bank of Washington and Hemisphere National Bank.