Abstract Next-generation vehicles which include Electric Vehicles (EV) and Hybrid Electric Vehicles (HEV) are researched and expected to reduce carbon dioxide (CO2) emissions in the future. In the national new-car sales in 2012 of Japan, the total sales of hybrid vehicles kept 26.5% share. In the field of passenger cars, this share was 29.7%. And, this share rose about four times compared to that of 2008 [ 1]. Also, small delivery hybrid trucks are increased in the commercial vehicle class. Fuel economy of hybrid trucks in the catalog specifications is relatively better than that of the diesel tracks which have no hybrid systems. Nevertheless, hybrid trucks' users report that advantages of fuel economy of hybrid trucks at the real traffic driving conditions are small. In this report, in order to research that the actual traveling fuel economy of hybrid truck' users has no advantage compared with the diesel truck' users, the traveling fuel economy of hybrid trucks and diesel trucks was surveyed by using the chassis dynamometer system. Then, the measure to improve the fuel economy of hybrid truck was identified by using the hybrid power-train bench test system. 1. Chassis dynamo meter testTest vehicles of hybrid truck and diesel truck are suited the new long-term emission regulation. Test vehicles were set 4, 135kg. Fuel economy of each vehicle was investigated by using chassis dynamometer system. Improvement of fuel economy of hybrid truck was compared to diesel truck at JE05 mode, Urban, Rural and Highway. The test result, improvement of fuel economy of this hybrid trucks in the urban was low. And, when average vehicle speed was high, improvement of fuel economy of hybrid truck was increase. This result was equal to the data of hybrid trucks' users. For one thing, driving speed of hybrid trucks' users was low. In other words, this driving pattern was similar to urban mode. So, hybrid trucks could not get regeneration power in this urban mode. Thus, these trucks could not assist engine power by electrical-motor. 2. Hybrid power-train bench test Control methods of improved fuel economy of hybrid trucks were investigated by using HEV power-train bench test system, which is possible to reproduce virtual HEV in the test room without producing a prototype of HEV. Control methods of improved fuel economy at JE05 urban mode were investigated. As the result, Improvement of fuel economy of this truck, which was operated at the optimal engine operation line, was 29.4% compared to diesel truck. This engine was operated at high efficiency conditions, which is in lower engine speed and higher engine torque. At the same time, electrical-motor of this truck was operated the production power or the regeneration power. Introduction Recently, climate change due to global warming has become one of major social problems, and increase in the amount of carbon dioxide (CO2) in automobile emission is highlighted as one of factors responsible for such increase. Automobile manufacturers are engaged in research and development of next-generation vehicles, such as hybrid vehicle, electric vehicle, etc., positively toward reduction of the carbon dioxide emission. In the national new-car sales in 2012 of Japan, the total sales of hybrid vehicles kept 26.5% share. In the field of passenger cars, this share was 29.7%. And, this share rose about four times compared to that of 2008 [ 1]. In the commercial vehicle class, market introduction by the hybrid truck has advanced [ 2]. However, the businesses having introduced small hybrid trucks for delivery (hereinafter called, the “hybrid trucks”) reported that the hybrid trucks lack a competitive advantage in terms of fuel economy compared to A Study of Fuel Economy Improvement of Small Delivery Hybrid Trucks2014-01-2903 Published 10/13/2014 Nobunori Okui and Tetsuya Niikuni National Traffic Safety & Enviro Lab. CITATION: Okui, N. and Niikuni, T., "A Study of Fuel Economy Impro