Perhaps showing the roots of a prolonged trend, Black Book reported that its latest Used Vehicle Retention Index — calculated using the company’s published wholesale average value on two- to six-year-old used vehicles as percent of original typically equipped MSRP — stood at 113.0 at the end of March.

The reading represented a 0.8-percent decrease from the previous month’s mark of 113.9.

Black Book explained that rising interest rates and auto finance delinquencies have led to a slight tightening of credit availability. Editors also pointed out that new-vehicle incentives continue to increase, and certified pre-owned growth has continued to slow, all creating downward pressure on retention values, resulting in a continued easing downward of the monthly Index.

The index is weighted based on vehicle registration volume and adjusted for seasonality, vehicle age, mileage, condition and inflation.

Two individual segments showed significantly higher drops according to their index calculations. They included sub-compact cars (down 2.2 percent) and compact crossovers (down 1.6 percent).

“We are continuing to see that the market is steadily declining, and a typical four-year-old vehicle is not retaining the same value over time,” said Anil Goyal, senior vice president of automotive valuation and analytics for Black Book.

“Given the direction of supply, demand, incentives and interest rates, we feel this easing downward trend for the index will continue throughout the balance of 2017,” Goyal continued.

The index dates back to January 2005, where Black Book published a benchmark index value of 100.0 for the market. During 2008, the index dropped by 14.1 percent while during 2016, the index by softened by 6.4 percent.

During 2011, the index rose strongly from 113.3 to 123.0 by the end of the year as the economy picked up steam and used vehicle values rose higher. It continued to remain relatively stable, rising slightly till May of 2014 when it hit a peak of 128.1. That being said, the index has fallen steadily since May 2015, when the Index registered a score of 125.9.

Black Book reiterated this recent trend illustrates a continued, slow weakening of the used-vehicle market as a result of cresting demand and increased supply in the used market.

To obtain a copy of the latest Black Book Wholesale Value Index, go to this website.