For decades, the medium- and heavy-duty (“MD/HD”) commercial vehicle industry has focused on improving freight efficiency in order to lower customers' total operating costs. To optimize fuel efficiency, most manufacturers no longer focus on discreet components but instead look at the complete vehicle and operations. The path to future efficiency gains is not sufficiently clear when looking towards 2030; what is clear is that one solution will not work for all manufacturers or vehicle applications. Therefore, fuel efficiency regulations must be sufficiently adaptive to allow a variety of technical approaches to ensure the needs of the commercial truck market are met. This paper explores further the ideas presented in other papers that focus on regulation of engine-only emissions as an approach for HD vehicles. This paper will describe EPA's Phase 1 Greenhouse Gas Emissions Model (“GEM”) and discuss technologies expected to be used to improve fuel efficiency in the future and how the next generation of GEM (“NextGenGEM”) for Phase 2 should be improved to capture these technologies, including why drive cycles should be revised. This work will also discuss how an improved NextGenGEM simulation can enhance the regulation's ability to be technology-neutral, align with real-world advancements, properly reflect technology penetration with real-world GHG reductions, result in customer acceptance, and maintain a reasonable regulatory burden that works with the product diversity that defines the HD market without compromising emissions compliance. Finally, this paper will evaluate ideas to improve the effectiveness of future GHG regulations for the HD market and ensure regulatory integrity.