Most American consumers do not understand credit scores and how they affect mortgage rates, according to a survey released this week by national lender GMAC Mortgage Corp.

The survey mirrors earlier findings by the Consumer Federation of America and Fair Isaac Corp., creator of the FICO score used by most lenders to evaluate those seeking credit.

GMAC's May survey of 1,057 households showed that:

* 62 percent didn't know that a score of more than 620 out of 850 is needed to get the best mortgage rate.

* More than 50 percent incorrectly said higher income would raise a score.

* Only 42 percent knew that payment history on debts is a critical factor.

A credit score is a number that lenders use to predict if a person will pay on time. FICO scores range from 300 to 850; most people score in the 600s and 700s. Scores below 600 indicate high risk and could lead to higher rates or to rejection.

A 720 score now means a 30-year rate with about 5.5 percent interest, while a 580 score means 8.5 percent or more. On a $100,000 loan, the difference would be about $2,400 a year, or $72,000 over 30 years.

For the free guide, "Your Credit Scores," call the Federal Citizen Information Center at 888-878-3256 or write to Credit Scores, Pueblo, Colo. 81009. The brochure is available online at www.pueblo.gsa.gov.

-- Sandra Fleishman