Battery-electric heavy-duty trucks already cut tank-to-wheel energy demand by roughly two thirds relative to diesel tractors; even so, their business case depends on squeezing out every additional percentage point of drivetrain efficiency. This study estimates the potential benefit of pairing a three-level (3L) traction inverter with multi-speed gearboxes in a 40 t European truck. A high-fidelity simulation of a representative long-haul mission profile, supplied with finite-element motor loss maps, physics-based inverter sub-models, and empirically derived gearbox efficiencies, indicates that replacing a two-level (2L)-six halfbridge (B6) inverter with a 3L T-type neutral point clamped (TNPC) inverter design alone lowers cycle energy use by approximately 4–6  \({}^{\text {kWh}}\!/\!_{\text {100\,km}}\) (about 4.1 %). Incorporating a two-speed gearbox trims an additional 1.9  \({}^{\text {kWh}}\!/\!_{\text {100\,km}}\) , whereas a four-speed variant secures a further reduction of 0.9  \({}^{\text {kWh}}\!/\!_{\text {100\,km}}\) , raising the total improvement to 7 %. The analysis therefore points to the 3L inverter as the chief lever, with extra gear steps offering modest yet valuable savings during long-haul duty cycles.