Dual loop EGR systems (having both a high pressure loop EGR and a low pressure loop EGR) have been successfully applied to multiple light-duty diesel engines to meet Tier 2 Bin 5 and Euro 5/6 emissions regulations [1, 2], including the 2009 model year VW Jetta 2.0TDI. Hyundai and Toyota also published their studies with dual loop EGR systems [3, 4]. More interest exists on the low pressure loop EGR effects on medium to heavy duty applications . Since the duty cycles of light duty diesel and heavy duty diesel applications are very different, how to apply the dual loop EGR systems to heavy duty applications and understanding their limitations are less documented and published. As a specific type of heavy duty application, this paper studied the dual loop EGR effects on the retrofit applications of heavy duty diesel for delivery and drayage applications. The reduction of NOx emissions and the impact on fuel economy and controls are discussed. The dual loop EGR systems were fully developed and demonstrated over the full engine speed and load range including transient conditions with a nearly 50% NOx reduction over light to medium loads for drayage truck applications relative to the 2004 emissions level. The complete dual loop EGR systems were implemented on the target vehicle for mileage accumulation and durability demonstration for future verification purpose.