Traditional fixed displacement oil pumps often have higher pumping capacity than the actual demand in most engine operating conditions, leading to additional energy losses. A variable displacement oil pump can dynamically adjust the effective displacement to reduce pump drive power and improve fuel savings. The experiment was conducted on a 2.5 L diesel engine to test the fuel saving potential of a variable displacement oil pump. Prior to the experiment, the oil pressure demand from various components was calculated, and the control characteristics of the variable displacement oil pump were evaluated. Based on the data, optimal oil pressure control targets were established. The comparative tests between the variable displacement oil pump and simulated fixed displacement oil pump were then conducted on an engine performance test bench. The results indicated that the reverse drag torque of the variable displacement oil pump decreased and obtained a reduction of approximately 2.5 N·m particularly under urban driving conditions. Additionally, the overall fuel consumption of the engine reduced, with the lowest specific fuel consumption reduced by 4 g/(kW·h). The vehicle tests for constant speed fuel consumption and China Cycle fuel consumption were conducted on a chassis dynamometer. The results indicate that, compared to fixed displacement oil pumps, variable displacement oil pumps offer significant fuel economy benefits at all vehicle speeds, although this advantage diminishes with the increase of vehicle speed. Notably, during the China Cycle fuel consumption test, the variable displacement oil pump exhibits a clear fuel consumption advantage in the urban segment, while the benefit is less pronounced in the suburban and highway segments. In summary, the precise control of oil pressure regulation enabled by the variable displacement oil pump substantially enhances engine fuel economy, with particularly notable fuel improvements under low-load and urban driving conditions.