SOUTHFIELD — An analysis by Polk revealed that the average length of ownership of vehicles that were purchased new has risen to a record 71.4 months, or nearly six years. For consumers who purchased used vehicles, the average length of ownership is nearly 49.9 months.

Combined, new and used vehicle owners are holding on to their vehicles for an average 57 months, according to Polk. For new and used owners combined, the length of vehicle ownership among U.S. consumers has increased 23 percent since the third quarter of 2008.

A number of factors contribute to the increased length of ownership, according to Polk, which analyzed vehicle registration data through Sept. 2011. First, consumer spending remains conservative in a still-weak job market with relatively high unemployment rates. Second, many buyers have longer-term financing options to secure more affordable payments. Third, vehicles produced in recent years have been more durable and more reliable than their predecessors, according to different industry reports.

Several manufacturers also are offering longer warranties for new vehicles, reducing the risk for consumers who want to keep vehicles longer.

Polk’s findings, coupled with the increased average age of vehicles on the road, which now stands at 10.8 years for cars and light trucks combined, offer promise for the automotive aftermarket. “As the aftermarket prepares to service this aging vehicle population, this creates concerns about appropriate parts inventory,” said Mark Seng, global aftermarket practice leader at Polk. “As a result of our analysis, we’re currently working with customers in the aftermarket to help them prepare for increasing demand throughout the entire supply chain."

Polk analysts do not anticipate new-vehicle sales will reach pre-downturn levels of 16 million units until 2015, and Polk does not expect to see an immediate decline in the length of ownership trend over the next few years, according to Seng. “Unemployment rates continue to be high, and we expect many consumers will suffer from the lingering effects of the downturn, further contributing to longer ownership trends,” he said.