This paper compares two structural options for the next phase of U.S. heavy-duty vehicle fuel efficiency and greenhouse gas standards. The program could regulate engines and vehicles separately, as the first phase did, or it could move to a full-vehicle standard that reflects engine performance. We evaluate the feasibility and efficacy of the two approaches relative to six criteria. We also present variations of these approaches, intended to take advantage of the benefits of each while avoiding the pitfalls. For each topic discussed, the paper includes questions to manufacturers and others who can provide additional technical information. We conclude with a preliminary recommendation that the Phase 2 standards combine the two structural options.