CO, NOx and HC emissions measured for the two motorcycles throughout ECE?EUDC and WMTC driving cycles are reported in Fig.2. The mean emissions obtained of the regulated pollutant are expressed as mass emitted per kilometer travelled and are presented for all the phases and for the whole DCs. It is clear in this figure that the two motorcycles fulfill the European statutory emission requirements for all regulated pollutants. The motorcycle B, however, shows higher emissions of CO and HC than motorcycle A, and this difference could be imputable not only to the catalytic converter efficiency, but also to partial combustion in driving situations characterized by rapid steep increase in engine speed that is no longer compensated by the catalytic converter of the vehicle B. A rich air– fuel mix is thus supposed often to be provided to the combustion process of the motorcycle B in such driving conditions, which causes the highest emissions of CO observed (Zamboni et al.2011), while it is well known that a three-way catalytic converter requires conditions of lambda very close to one permitting the oxidation of CO and HC and at the same time a reduction of NOXto elemental nitrogen. It appears, therefore, that the fuel management system and the capacity of the catalytic converter have not been designed suitably for the motorcycle B, as well as, on the contrary, can be deduced for the vehicle A. A lessening enrichment, for the motorcycle A, is probably the effect of internal engine improvement and more correct mixture control of fuel injection systems used on these vehicles, consenting a better control of fuel feeding and rising catalyst efficiency (Alvarez et al. 2009).