Auto lenders continue to defy warnings about growing risks in the market.

During the fourth quarter of 2015, U.S. auto loans carried longer terms, while average monthly payments rose, and a larger percentage of loans went to less creditworthy borrowers, according to a new report from Experian Automotive.

New car buyers borrowed an average of $29,551 to finance their purchases, up from $28,381 during the fourth quarter of 2014, the report found.

Used-car loans averaged $18,850, which was up from $18,411 a year earlier.

To keep monthly payments affordable, auto lenders have been offering longer terms to car shoppers. That worries some observers who believe that used-car prices have been at unsustainably high levels in recent years. If the values of used cars drop, lenders will suffer larger losses on longer-term loans than they do on loans with shorter repayment schedules.

In the fourth quarter of 2015, the average term for a new car loan reached 67 months, up from 66 months a year earlier, according to Experian. For used cars, the average loan term rose from 62 months to 63 months.

Even with longer loan terms, borrowers are still reaching deeper into their pockets each month. Average monthly payments on new cars rose to $493 in the fourth quarter, up from $482 a year earlier. For used cars, average monthly payments increased from $355 to $359, the report found.

In addition, the biggest pockets of growth in the auto lending industry during 2015 came in the riskiest segments, according to Experian.

Loans to borrowers with the lowest credit scores, between 300 and 500 on the VantageScore 3.0 model, rose by 14.7% from the fourth quarter of 2014. For auto buyers in the highest band of credit scores, above 780, loan volume grew by 8.06%.

Signals about credit quality in the auto-lending business were mixed in the fourth quarter, according to the Experian report.

The percentage of loans that were at least 30 days delinquent fell from 2.62% to 2.57%. Meanwhile, the percentage of loans that were at least 60 days past due rose from 0.72% to 0.77%.

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