Black Book saw its Used Vehicle Retention Index fall in December to 114.1 from 115, its first decline since August. 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. The index offers an accurate, unbiased view of the strength of today’s used vehicle market values.After rising for three straight months, the index fell largely due to higher depreciation recorded for both car and truck segments. December saw higher depreciation on a mix of both car and truck segments, led by full-size luxury CUV/SUV (2 percent); mid-size luxury CUV/SUV (1.9 percent); full-size cars (1.7 percent); compact crossovers (1.7 percent) and compact cars (1.4 percent).