Development of transportation fleet based on electric vehicles (EVs) is one of the most important approaches proposed for preserving environmental sustainability. Therefore, considering the efficiency of EVs has become to one of the hot topics in energy management systems. In this regard, electric energy storage systems might have the most significant impact on increasing the efficiency of EVs as well as the traveled distance. Using a supercapacitor as an auxiliary source supports the batteries in proper power management. Among the important features of supercapacitors, less internal resistance and higher power density compared to batteries can be mentioned. However, their energy density is lower than batteries. Also, supercapacitors have a sufficient lifetime with high efficiency and fast response for electrostatic charge/discharge. In this paper, a combination of supercapacitors and batteries is used to improve the EV's performance and mileage. In addition, an online power management method based on the fuzzy logic is proposed considering terrain inaccuracy. The simulations are done using MATLAB to validate the energy efficiency improvement of the EV's battery.