Wireless communications systems are increasingly demanding energy-efficient solutions to address the growing environmental concerns and the need for sustainable technologies. This paper investigates the energy efficiency gains achievable with a Gearbox-physical layer (PHY), capable of dynamically switching between different modulation schemes and corresponding front ends, denoted as gears. Utilizing models and literature-based measurements of the power consumption of key analog front end components, we extend our previous work on the energy saving potential of a Gearbox-PHY by considering time-domain modulation schemes, i.e., zero-crossing modulation (ZXM) and time-derived ZXM (TD-ZXM), in addition to quadrature amplitude modulation (QAM). This also includes an evaluation of the impact of these different classes of modulation schemes on the efficiency of the power amplifier. From an energy efficiency perspective, our analysis reveals that QAM is optimal for peak data rates and low carrier frequencies, while ZXM is preferable for medium to high rates, and TD-ZXM excels at low rates. Comparing the Gearbox-PHY approach with the conventional single-gear approach shows that significant energy savings are possible, suggesting a paradigm shift in focus from peak spectral efficiency to energy efficiency in PHY design.