Black Book reports that the average price of a used vehicle for model years 2013 to 2017 decreased in value by 1% in September, with car values declining by 1.1% and trucks down 1%. All vehicles are averaging a 12-month depreciation decline of 13.9%.

Subcompact crossovers (0.4%) were the only vehicle segment that saw a positive change last month; though full-size luxury CUVs and SUVs were essentially flat (down 0.1%).

Compact vans (down 2.2%) and luxury cars (down 1.9%) suffered the greatest declines.

"September has come in strong in the last couple of years. However, the values were softer in the month this year, consistent with previous seasonality trends," said Anil Goyal, executive vice president of operations at Black Book. "The used vehicle values could see a steeper decline in Q4 if the demand slows as inventories on both new and used vehicles remain high."