Nissan Motor Acceptance Corporation (NMAC) on July 11 released a report that said it "strongly refuted" two recent studies indicating that black customers had paid more than whites to finance cars through Nissan dealerships.

The report, by a research firm hired by Nissan, said the earlier studies, by two professors hired by black consumers suing NMAC, ignored credit differences and variations in loan terms that might explain patterns in financing costs.

According to the Nissan report, the sample the professors relied on — believed by experts to be the largest pool of car loan data ever analyzed for racial patterns — was too small and too geographically skewed to be fairly representative of Nissan's lending practices.

The professors studied 310,000 of the 1.1 million loans made by NMAC since 1993 — all those on which they found racial information.

"After a critical review, NMAC has determined that the reports commissioned by the plaintiffs used seriously flawed methodology to reach a contrived conclusion," Nissan said in a statement.

The company's report was filed in Federal District Court in Nashville on July 10, a spokeswoman said.

Neither the lawsuit against NMAC nor similar cases against the lending units of other major auto dealers including General Motors Acceptance Corp. (GMAC) accuse the lenders themselves of racial bias. Lawyers for the black consumers argue, however, that lenders engage in loan-pricing arrangements with dealers that result in blacks paying more to finance cars.

Lenders like NMAC routinely allow dealers to add several percentage points -- known as the dealer markup -- to the interest rate the lender has established for customers based on income and credit history. That becomes the rate the dealer quotes to the customer, who normally has no way of learning that the lender approved his loan at a lower rate. The lender splits the money generated by the extra percentage points with the dealer.

After analyzing the amount of extra interest that dealers added to loans financed by NMAC, Prof. Mark A. Cohen of Vanderbilt University concluded that blacks were more likely than whites to have paid a dealer markup and that blacks paid larger markups than whites, regardless of their credit history.

Prof. Ian Ayres, a lawyer and economist at Yale Law School, endorsed Professor Cohen's findings and suggested that NMAC might inadvertently be contributing to the disparate pattern by limiting the markup on certain loans that are more likely to have been made to whites.

Janet Thornton, a labor economist at Economic Research Services in Tallahassee, Fla., who reviewed the two reports for Nissan, argued that both professors "have a flawed understanding of the actual markets for vehicles and credit." According to Thornton, Professor Cohen relied on a small, unrepresentative sample and ignored the effect of credit history on the dealer markup.

Clint Watkins, a lawyer for the plaintiffs, said the professors stood by their conclusions.