Rising car depreciation put a drag on used vehicle sales, which fell in value significantly more sharply in August compared to July and August from a year ago, according to Black Book.

Vehicles from the 2011 to 2015 model years depreciated 2.4% compared to the 1.5% rate in July. Depreciation rates for the last three years of August were 1.4% for 2015, 1.9% for 2014, and 1.3% for 2013.

Car categories depreciated 3.1% in August, while truck categories depreciated 1.8%. Among the car categories, entry luxury cars depreciated 3.7% to $18,112 and declined 19.5% from a year ago. This category includes the Audi A4, BMW 3-Series, Cadillac ATS, and Lexus IS250.

Full-size pickups registered the strongest retention rate by depreciating only 0.6% to $24,372. These vehicles fell 10.2% in value from a year ago.

"As fall approaches, depreciation trends continue to accelerate as we head toward the last few months of the year," said Anil Goyal, senior vice president of automotive valuation and analytics. "We can expect to see lower retention in cars across all segments as the year continues, and we'll continue to monitor truck levels to see if they also see weaker retention overall."

Other segments with weak retention included subcompact cars (3.5% to $6,970), sporty cars (3.4% to $14,951), luxury cars (3.3% to $22,517), and compact cars (3.3% to $8,737). Segments with stronger retention included subcompact luxury CUVs (1.5% to $18,242, small pickups (1.5% to $19,809), and full-size vans (1.6% to $19,823).