A great deal of work has been done in recent years to measure or to predict the real-time emissions from motor vehicles. This is necessary not only to control and to reduce the emissions but also to calculate the real emission load resulting from motor vehicles. Global measurements made on representative driving cycles or calculations made with global emission factors do not reflect the real emission characteristics of vehicles on the road. in this paper, a theoretical approach is demonstrated for the calculation of the CO emissions of a vehicle powered by a gasoline engine. The calculation model is based on the air excess ratio (lambda) records of the engine while following a driving cycle. Other engine parameters, such as engine speed, ignition time advance, throttle valve position, intake air and coolant temperatures, fuel and air flow-rates, are also recorded to feed the thermodynamic cycle and CO emission calculation models. The calculated and the measured total CO emissions at the end of the driving cycle are approximately at the same level, although instantaneous emissions are not coincident quantitatively well on the time axis.