Analyzes the implications of a growing natural gas vehicle fleet on the emission benefits of the U.S. HDV “Phase 2” greenhouse gas rulemaking, synthesizing data on upstream emissions, vehicle emissions, and efficiency technology. Recent assessments indicate that natural gas vehicles could constitute up to 20 percent of the new 2025 heavy-duty vehicle market, potentially displacing a significant number of diesel vehicles. Considering the relative lack of regulatory guidance at this stage, there are many ways in which the U.S. heavy-duty vehicle “Phase 2” greenhouse gas rulemaking could influence the new fuel/engine technology and its deployment in the fleet. The possibilities range from provisions that promote natural gas use in trucks, with minimal protection against increased methane emissions, to those that promote only the lowest-carbon natural gas technology and can provide even greater carbon and oil-saving benefits.