Black Book's Used Vehicle Retention Index fell to 114.0 in November, a 1.4% decrease from October (115.6). The index would appear to have peaked in August (116.6); in 2018, it rose from 114.1 in January to 116.3 in December — nearly 2% higher than last month's figure.

The report found that gains among sporty cars (0.43%), compact cars (0.34%), and midsize cars (0.22%) kept retained values essentially flat compared with September (115.9), even as several truck categories showed losses.

"The index has been relatively stable over the last two years. However, the combination of easing demand, high levels of inventory, [and] increased incentives on new vehicles has led to additional softness beyond seasonal decline in the fourth quarter for the used vehicle market," said Anil Goyal, Black Book's executive vice president of operations.

The Black Book Used Vehicle Retention Index is calculated using Black Book's published Wholesale Average value on two- to six-year-old used vehicles, as percent of original typically equipped MSRP. It is weighted based on registration volume and adjusted for seasonality, vehicle age, mileage, and condition.