A dynamic wireless charging system for electric vehicles (EVs) is expected to extend the limited driving distance of EVs. As the transmitting efficiency changes according to motion of the vehicle in dynamic charging, an efficiency maximization method is important. Previous research has proposed secondary-side efficiency control based on mutual inductance estimation to simplify the ground facilities, which would be installed over long distances. However, the ground facilities have to regulate the primary voltage to achieve maximum efficiency control on the secondary side without signal communication. In this paper, a calculation method of the reference value for maximum efficiency control is proposed using simultaneous estimation of the primary voltage and the mutual inductance on the secondary side to eliminate the need for the primary voltage regulation. Simulations and experiments demonstrate that the proposed method is available for maximum efficiency control on the secondary side.