This study proposes a drivetrain design space optimization methodology for a battery electric vehicle equipped with a permanent magnet synchronous motor (PMSM) and a single-speed transmission. The approach integrates a longitudinal vehicle dynamics model, PMSM efficiency and current maps derived from finite-element electromagnetic simulations, drivetrain feasibility constraints, and an Efficiency Map Utilization Framework (EMUF). The proposed framework introduces energy-based descriptors to characterize how the PMSM efficiency map is utilized during traction and regenerative operation, complementing conventional vehicle-level energy metrics. The methodology is applied over the WLTC Class 3b cycle to investigate the combined influence of transmission ratio and PMSM capability.  Under the investigated vehicle, the minimum consumption feasible configuration is obtained at a transmission ratio of 4.25 and a PMSM capability factor of 0.90, yielding 13.489 kWh/100 km. The results show that drivetrain feasibility and energy performance are strongly related to the distribution of PMSM operating points across the efficiency map, while traction and regenerative utilization may exhibit different optimal trends. EMUF therefore provides a complementary physical interpretation of drivetrain energy performance and supports the comparison of feasible drivetrain configurations.