The first fuel efficiency and greenhouse gas standards for heavy-duty vehicles in the United States, adopted in 2011, established an approach to testing and regulating these vehicles that is essentially component-based. Capturing additional fuel efficiency opportunities in the next phase of standards will require a more integrated approach, tailored to the wide array of vehicles regulated under the standards. Detailed data on the specifications and use of heavy-duty vehicles will be essential to this undertaking. While implementation of the first phase of the standards will yield useful data on new vehicles, important gaps will remain, and the availability of much of that data to the public is uncertain. For in-use vehicles, despite several important ongoing data collection and analysis efforts, a comprehensive picture of the current heavy-duty stock in the United States is lacking.