Nowadays, reducing frictional loss of gears to save energy is a big concern. There are many sources of printed information on this problem for gears, but most of them are practical specialized information and are not developed to give a generalized basic consideration. Since this kind of research has not been brought beyond such, a practical and specialized range, experiments were carried out to study the influences of tooth form, sliding velocity, load, tooth surface roughness, and viscosity of lubricating oils, and kinds of oils on the frictional loss of gears. The frictional loss of spur gears decreases with sliding velocity and the viscosity of lubricating oils, and increases with surface roughness. The frictional loss is very low when using a synthetic oil of the SHF type. The organic molybdenum friction modifier (FM) is also quite effective for reducing frictional loss. Finally, empirical formulas for calculating frictional loss were expressed in terms of tooth form, sliding velocity, tooth surface roughness, viscosity of the lubricating oil and load.Presented at the 45th Annual Meeting in Denver, Colorado May 7–10, 1990