A remarkable amount of fuel consumption is due to driver's incorrect or non-optimized shift behavior. To solve the problem, a shifting guidance system based on the best fuel economy is proposed. The basic principle is to read the vehicle state parameters via OBD-Ⅱinterfaces, and then to remind the driver shift at the right time based on the optimal shift theory. The design and implementation of the system is detailed, including the system framework, missing parameter identification algorithm, the optimal shift discrimination algorithms. Finally, the system validation results show that the application of the system can bring an average fuel savings of 8.6%, and can make the fuel consumption more stable. Results of this study can be generalized in reality, and has an essential value for energy saving and emissions reduction of transportation.