Studies have shown that lubricating oil has some influence on the efficiency of the transmission gearbox.By simulating the vehicle combined cycle mode according to GB/T 12545.5-2008,the vehicle simulation test was performed to study the influence of transmission gearbox lubricating oil on the vehicle fuel consumption at combined cycle mode.The test result verified that low-viscosity oil has a positive effect on improving the efficiency of transmission gearbox and vehicle,and reducing vehicle fuel consumption.The experimental results also show that transmission oil of different viscosity has different effects on vehicle fuel consumption, and the low-viscosity oil of transmission gearbox can effectively reduce the vehicle fuel consumption.