NEW YORK -- Reports of lenders botching mortgage-rate calculations have sent homeowners scrambling to see if they are being overcharged -- and left bankers wondering if they are losing millions of dollars in revenue.

The differing views on adjustable-rate mortgages -- loans with variable interest rates -- began when a former federal banking auditor estimated that there were widespread errors in the loan portfolios of failed savings and loans in the Midwest.

John Geddes, formerly with the Federal Savings and Loan Insurance Corp., the thrift deposit insurance agency whose functions have been transferred to the Federal Deposit Insurance Corp., has estimated that up to 35 percent of adjustable-rate mortgages nationwide were miscalculated and that borrowers were overcharged $8 billion from 1979 to 1989.

There haven't been any more studies to confirm Geddes's statements, and officials are unsure about how widespread the miscalculations may be.

"Clearly the portfolio that we were looking at is the smaller savings and loans in the Midwest. But whether the problem is broader than that, I don't know," said Steve Seelig of the FDIC, the federal fund that insures bank deposits.

At any rate, Geddes's charges in August sent shockwaves through the lending community. Lawyers in Indiana have filed at least four class-action lawsuits alleging that S&Ls excessively overcharged borrowers.

The General Accounting Office, the investigative division of Congress, interviewed experts about the problem and issued a report on the issue in October. Federal auditors now pay special attention to the controversial loans when reviewing the financial records of banks, credit unions and S&Ls, said Mitch Rachlis, senior economist for the GAO.

Meanwhile, an umbrella group of federal financial services regulators, the Federal Financial Institutions Examination Council, is coordinating an examination of banks, savings and loans, and credit unions that have adjustable-rate mortgages in their portfolios.

The move by the council shows that the government is taking the problem very seriously, said Fritz Elmendorf, spokesman for the Consumer Bankers Association in Arlington.

So, apparently, are homeowners. HSH Associates, a New Jersey mortgage information service, is marketing kits for borrowers to double-check their mortgage calculations.

"We have got probably 400 phone calls in the last couple of days, and that would be a conservative guess," HSH vice president Paul Havemann said in a recent interview.

An adjustable-rate mortgage, or ARM, has an interest rate that fluctuates according to a financial index, such as a Treasury bill. A borrower also will pay a margin, typically 2 to 2.5 percentage points, above the index.

ARMs have limits on the total amount of fluctuation in a given time period. The mortgages are attractive during periods of high interest rates because their rates are lower than standard fixed-rate mortgages.

Errors may arise when a lender mistakenly uses the wrong index, calculates the index at an improper time or lacks proper computer software to tabulate the mortgage. Incorrect rounding of numbers also is blamed in some instances.

George Plews, an Indianapolis lawyer, said he is working with lawyers in 15 other states whose clients have problems with their ARMs. Plews, who has filed the four class-action lawsuits in Indiana, said a typical loss ranges from $200 to $1,400 during the term of the loan.

"Our information is that this is not a new problem in the industry," Plews said. "The inaccuracies in the portfolio were relative common knowledge in the late 1980s" among bank regulators. Plews has been working closely with former bank regulator Geddes on the issue.

Plews said his review shows the problem afflicts banks as well as savings and loans.

An American Bankers Association spokeswoman disagreed, saying the problem now appears to be confined to just savings and loans.

Association spokeswoman Tara Little said some lenders, such as Chase Home Mortgage in Tampa, reviewed their portfolio and found "almost no errors" in calculations.

"Adjustable-rate mortgages are very complicated financial instruments," she said. "Basically, there is human error that you can decrease by automating and using a software system."

The association is providing a new software package to help bankers properly calculate the mortgages. Little encouraged any consumers with doubts about their mortgages to ask their lender for an explanation.

She and other industry officials said they were unaware of any consumers avoiding ARMs in light of the recent publicity.

Some analysts say the bulk of the errors may even be in consumers' favor, while others say the damage is evenly split.

"The adjustable-rate mortgage problem is more a problem of complexity as opposed to a problem of greed," said Seelig, who is the FDIC's liquidation division director.

If a mistake winds up being in a homeowner's favor, it is unlikely the borrower will be forced to make up the balance, Seelig said.