The average interest rate on a 30-year fixed-rate mortgage was 4.58% in September, a drop of 12 basis points from August, according to the Federal Housing Finance Agency. The FHFA calculates the average monthly rates for more than 5,100 loans closed by 42 lenders between Sept. 24 and Sept. 30. On those transactions, the interest rate is determined 30 to 45 days before the loan is closed. The FHFA’s index for the average rate on the purchase of previously owned homes fell 10 bps to 4.55% for the month, another new low since 1980 when mortgage rates were as high as 15%. The average interest rate on a 15-year fixed-rate mortgage did increase 11 bps to 4.57%. The contract rate on the composite of all mortgage loans, both ARMs and FRMs, fell to 4.52% from 4.63% the month before. Write to Jon Prior.