2014 and 2015 were marked with the high truck build rates, which means that in the next few years the market will be flooded with 3-5 year old tractors. This will put pressure down on used trucks prices.
The current supply of used trucks is determined on what was retailed 3-5 years ago. For instance, in July this year, 5 year old (2011) trucks were down 15% comparing with July 2015, and 3 year old trucks (2013) were down 18%.
According to experts, the amount of trucks that will come back as used trucks will keep growing to peak around 2020. Increased supply will also have its effect on pricing.