We carry out a comparative study of energy consumption of the conventional internal combustion truck and the modern electric truck, traveling from origin to destination over the national highway subject to a hard deadline. We focus on understanding energy saving of the latter over the former and key contributing factors. Our study is unique in that (i) it is based on extensive simulations using real-world data over the U.S. highway system, and (ii) we factor in the power system energy-conversion efficiency when calculating the energy consumption of electric trucks for fair comparison. The results show that on average the electric truck save 10% energy as compared to the internal combustion truck, and this saving will improve as power systems incorporate more renewable generation. Furthermore, the energy saving mainly comes from the energy efficiency of electric motors, and other electric-truck features, e.g., regenerative braking, only have minor contributions.