The wholesale used-vehicle market finished 2015 stronger than expected with an average 13.2% depreciation rate for used vehicles two to six years old and trucks retaining their value 9% better than passenger cars on average.

Cars depreciated 18.2% to $14,196, while light-duty trucks lost 9.2% of their value to $20,543 for the full year. The increasing gap in values has been driven by persistently lower gasoline prices that may widen in 2016, according to Black Book.

"Entering 2016, a record year for car sales totaling more than 17 million vehicles, the decrease in used car prices that we are seeing are showing a still-stable used-vehicle market," said Anil Goyal, Black Book's vice president of automotive valuation and analytics. "One thing to watch, however, is the discrepancy in depreciation patterns between car and truck segments, as this gap is widening."

In December, a historically slower month for used-vehicle sales, vehicle values declined 1.8% to $14,897. Cars fell 2%, while trucks fell 1.7%.

Full-size pickups led all segments with the lowest depreciation, falling 0.5% to $23,786 for the month.

Subcompact luxury CUVs including the BMW X1 showed the highest depreciation, falling 6% to $18,039. Other categories with a monthly depreciation of at least 3.2% included subcompact crossovers, and compact vans such as the Ford Transit Connect, Nissan NV200 and Ram C/V.

Annual depreciation came in lighter than the 14.5% forecasted in a February 2015 report by Black Book at Fitch Ratings, and was above the typical pre-recession range of 15% to 18% per year.

On the year, full-size SUVs such as the Chevrolet Tahoe and Ford Expedition lost the least value with a 0.9% decline to $25,378. Full-size pickups such as the Ford F-150, Chevrolet Silverado 1500, and Ram 1500 fell 1.7% in 2015. Other categories with single-digit depreciation included small pickups (4.2% down to $18,809) and full-size vans (down 5.7% to $20,068).

Compact vans showed the highest depreciation, falling 22.6% to $10,478. Other categories that recorded at least 20% depreciation during the year included subcompact cars (21.7% down to $7,229), compact cars (down 20.3% to $8,463), and subcompact luxury CUVs (down 20% to $18,039).

The report tracks mostly auction sales of vehicles in the 2010 to 2014 model years.