It has recently been demanded to improve fuel economy of heavy duty diesel trucks and buses. Advanced technologies such as downsizing and lower friction of diesel engines for better fuel economy have been being developed, and technologies of diesel engine oils for better fuel economy have been being improved. However, Automotive Diesel Engine Oil Standard (JASO M355:2015) which is applied in Japan as a standard for diesel engine oils, does not include any requirement of fuel economy. Then the task force team consisted of related industries have developed a fuel economy test method using Hino N04C engine which meets Japanese emission standard in 2010. The new test method measures fuel economy performances of heavy duty diesel engine oils for fresh oils and aged oils. Aged oils were prepared by JASO M336:2014 or JASO M354:2015 using the N04C engine. The detailed test protocol for the new fuel economy test method, which is based on the governmental test method for Heavy-duty Motor Vehicle Fuel Economy Test (TRIAS 5-8-2010). Fuel economy performances of candidate oils are evaluated by comparing to the reference oil having SAE 30 viscosity grade. The new test method can differentiate fuel economy performances of the three oils, which are same viscosity grade of SAE 5W-30. The difference could be caused by difference of viscosity properties. It was currently proposed that minimum criteria of improvement rate of fuel economy against the reference oil for fresh oil is 3.7%, and that the minimum criteria of the sum of the improvement rates for fresh and aged oil is 6.8%. These values were determined by statistical process. The revised JASO M355 adding the new category of fuel economy oil for heavy duty diesel engine is released in 2017.