The bacterial flagellar motor (BFM) is a highly efficient molecular motor that converts proton motive force (PMF) into rotational motion, enabling bacterial locomotion. While previous studies have extensively investigated the relationship between PMF, rotational speed, and torque under low to high load conditions using bead assays, the motor' s behavior under ultra-high load conditions has not been systematically explored. In this study, an ultra-high load condition was realized by tethering the bacterial flagellum to a glass surface, forcing the entire E. coli cell body to rotate. This configuration imposes a load exceeding that of conventional bead-based assays. PMF was regulated by varying glucose concentrations in the motility buffer (0 mM, 10 mM, and 20 mM), and bacterial rotation was recorded using phase-contrast video microscopy. Rotational frequency was extracted through image analysis, and motor torque was estimated by modeling the bacterial cel l as a rotating ellipsoid under low-Reynolds-number hydrodynamics. The results show that increasing glucose concentration significantly enhances rotational speed and torque from 0 mM to 10 mM glucose. However, further increasing glucose concentration to 20 mM does not produce a statistically significant increase in either rotational speed or torque. This indicates that, under ultra-high load conditions, the PMF-dependent enhancement of motor performance saturates beyond a certain threshold. Torque–speed analysis further reveals a linear positive correlation at low rotational frequencies (1–2 Hz), while deviations from linearity emerge at higher frequencies. These findings demonstrate that, under extreme load conditions, the relationship between PMF, rotational speed, and torque differs from the behaviors reported in previously studied load regimes. The results provide experimental evidence that ultra-high mechanical load imposes intrinsic constraints on the bacterial flagellar motor, extending current understanding of its load-dependent dynamics.