A recent study by Frost & Sullivan (summarized here – subscription required) forecasts increased new truck sales in upcoming years, potentially resulting in increased trade-ins and lower used truck pricing. One comment in the study caught my attention. Specifically, the study states that 2007-2009 emissions engines were “…tougher sells to fleets due to higher prices and proliferation of EPA-compliant, OEM-branded engines that are yet to prove themselves in the used truck market.” 

That sentiment may have been true when the engines were first introduced to the new truck market, but in the 6 years since these engines were introduced, a large number of trucks so equipped have cycled through the secondary market. Selling prices are well-established. NADA data shows 62% of sleeper tractors reported sold retail and 33% of trucks sold wholesale since January have been EPA 2007-spec. To us, EPA 2007 is old news. We’ve already been making valuation judgments on EPA 2010 trucks for well over a year. 
In addition, the article does not mention the impact of West Coast emissions regulations on used truck demand. A pre-DPF truck is very difficult to sell in California and other states with strict emissions regulations. As such, 2008 and newer trucks are basically the only option in that region. This factor will help to maintain a favorable supply/demand ratio nationwide.
In terms of supply, the historically low build rate of 2008-2010 model year trucks will shape the used truck market in upcoming years. 2010 in particular was essentially nonexistent, at just over 118,000 trucks built. It is true that an additional few hundred identical trucks introduced to the secondary market in a short period of time could place downward pressure on pricing. However, the supply/demand ratio for trucks with under 600,000 miles is currently heavily demand-weighted. This means the market has room to digest increased supply. The over-600K market is less favorable, but even here depreciation should be proportional to the degree of increased supply. A moderate increase in supply should result in moderately increased depreciation.
Bottom line - an increased supply of used trucks, if it happens, would logically place downward pressure on pricing. However, the increased supply would be almost entirely comprised of trucks with over 600,000 miles. As such, in that scenario, we would expect a moderate pullback in wholesale pricing, while retail pricing for trucks with under 600K would likely remain flat to mildly downward.