Depreciation among wholesale medium-duty trucks continued to rise through July and August, according to Black Book's September Specialty Markets report.

In August, the value of medium-duty trucks from 2015 to 2016 fell 0.5% compared to the month before. In July, wholesale values were down 0.4% month over month.

Depreciation was steeper for units from 2007-2014, as values for these vehicles in were down 0.7% month-over-month in August and July.

"This past month we experienced a bit more depreciation than we did in June. Auction prices continue to drop on most of the units we track. The increased supply of box trucks is one driver," the report stated.

Dealers reported that retail demand was stable in the past two months, Black Book added.  However, with post-Harvey cleanup efforts on the way, the firm expects demand for medium-duty trucks to increase in the coming months, as many of the vehicles in this segment are used in various forms of construction.

In addition, diesel prices have also been on the rise over the past few weeks. The national average for a gallon of diesel is $2.60, 11 cents more than the previous month and 23 cents more compared to the year before.

"Reports are indicating that fuel prices could increase in the near future. So, if you have a fleet of trucks or a tank at your facility, now is the time to call and get them filled up," the report stated.