An adaptive energy-efficient control allocation (A-EECA) is developed for planar motion control of electric ground vehicles (EGVs) with four in-wheel motors. Different from distribution processes in previous EECA designs , which needed to instantaneously solve nonlinear (even non-convex) optimization problems at each sampling time, this adaptive EECA approach can make distributed control actuation gradually converge to energy-optimal operating points. The consequent advantages are reflected in both low computational cost and free selections of initial conditions. Based on experimental data and some reasonable assumptions on the efficiencies of in-wheel motors, the adaptive EECA dictates different torque distributions to all the EGV wheels based on their different efficiencies. Simulation results of different maneuvers indicate that much less energy and computational cost are consumed when the adaptive EECA scheme is applied for controlling the planar motion of an EGV.