Optimized charge air cooling (or optimized aftercooling) applied to Cummins diesel engines can provide fuel economy, performance, and driveability benefits. The reliance of this concept upon the optimum performance of both the engine and the vehicle cooling system dictated evaluation of the total package while operating in a vehicle environment.Engine and vehicle performance were measured on-highway using a unique test trailer featuring an integral dynamometer and a computerized data acquisition system. Parameters measured included a variety of engine temperatures, pressures, and flows, as well as power required, engine speed, and road speed. Tests were conducted in a wide range of ambient temperatures at several speed and load conditions. On-highway, road load operation was also evaluated.