A diesel fuel composition is disclosed, as well as a method for reducing NOx and smoke emissions from a diesel engine at minimum fuel consumption which comprises adding to the diesel engine at least one diesel fuel or blending component for a diesel fuel which has a combination of a low T50 in the range of from 190.degree. C. to 280.degree. C., a high cetane number in the range of from 31 to 60, and optionally a high distillation curve slope in the range of from 58.degree. C. to 140.degree. C., which combination is effective to afford a combination of the lowest NO.sub.x and smoke emissions at the lowest fuel consumption at independent engine control values for the diesel engine that are optimum to afford production of a combination of the lowest NOx and smoke emissions at the lowest fuel consumption, whereby the NOx and smoke emissions from the diesel engine are reduced by at least 10% and 15%, respectively.