The results for the light duty and heavy duty diesel studies in the EPEFE have been compared, and areas of similarity and differences explored. Consistent with the intent to choose the widest possible range of engine technologies and vehicle configurations, the impact of the vehicle and engine sets on emissions was larger than that of the matrix of fuel properties. Vehicles showed a wide range in response to the fuel properties investigated. In some cases, the effects of fuel changes on emissions from the LD vehicle and HD engine sets were similar, but in other cases emissions changed in opposite directions in the two sets for the same fuel change. However, fuel changes that were beneficial for PM in LD vehicles were also positive for NOx and PM in HD engines. Differences between engine design features and test cycles were examined to investigate the reasons for these results.Fuel effects were generally larger in percentage terms for Light Duty Vehicles than for Heavy Duty Engines. The effects of fuel density in HD engines were explained by physical interactions between the fuel and the fuel injection system, but this mechanism explained only part of the variation in LD vehicles.