The need to assess the effect of exhaust gas emissions from heavy duty vehicles (buses and trucks) on emission inventories is urgent. Exhaust gas emissions measured during the fuel economy measurement test procedures that are used in different countries sometimes do not represent the in-use vehicle emissions. Since both local and imported vehicles are running on the roads, it is thought that studying the testing cycles of the major vehicle manufacturer countries is worthy. Standard vehicle testing cycles on chassis dynamometer from the United States, Canada, European Community Market, and Japan1 are considered in this study. Each of the tested cycles is categorized as either actual or synthesized cycle and its representativness of the observed driving patterns is investigated. A total of fourteen parameters are chosen to characterize any given driving cycle and the cycles under investigation were compared using these parameters. The Positive acceleration Kinetic Energy - Velocity (PKE-V) model is used to simulate the fuel consumption and exhaust gas emissions rate and these rates are then related to the different cycle events. Some of the factors affecting the data obtained from different cycles are discussed and open-end recommendations are suggested for global standard testing cycles for different vehicle categories.