Overall depreciation for used vehicles from the past five model years improved slightly from July as the average value fell 1.3 percent in the month compared to 1.5 percent in July, according to a monthly report from Black Book.

Domestic passenger cars saw the sharpest decline, falling 2.5 percent, while import cars fell 1.6 percent and domestic trucks fell 1.1 percent. August typically sees greater depreciation for used vehicles as new 2015-MY models begin arriving in the marketplace. Depreciation rates, especially with trucks, are expected to improve in the later part of the year, according to Ricky Beggs, Black Book's editorial director.

"The near-term outlook for trucks continues to show significant retention strength and these segments are clearly driving the industry as a whole,” Beggs said. "While the trucks are outperforming seasonal trends currently, we’re not surprised to see segments such as entry-level cars and entry mid-size cars showing the largest depreciation.”

Cargo minivans led all truck segments with the highest monthly depreciation at 3.3 percent. Vehicles in this category such as the Dodge Caravan, Ford Freestar, Dodge Grand Caravan, and Ford Transit sold for an average of $11,192 for a 14.7 percent decline from a year ago.

Full-size passenger vans retained the most value for the second month in a row by increasing 0.1 percent and selling for an average price of $18,136. This category includes the Mercedes-Benz/Dodge Sprinter, Ford E-Series, and Chevrolet Express/GMC Savana wagon models.

In August, entry-level cars saw the sharpest decline in value with a depreciation rate of 4.1 percent. These vehicles sold for an average price of $8,157, a 14.7 percent decline from a year ago. Vehicles in this category include the Chevrolet Aveo, Honda Fit, Kia Rio, Nissan Versa, and Toyota Yaris.

For the third straight month, premium sporty cars led all car segments with the highest retention rate by falling 1.1 percent in value to $48,151, a 12.3 percent decline from a year ago. This category includes the BMW 6-Series, Mercedes-Benz CL-Class, BMW M6, Audi R8, Audi S8, Mercedes-Benz SL-Class, and Cadillac XLR.

The Black Book report tracks vehicles sold at auction from the model years 2009-2013.