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Abstract
Transportation systems contribute significantly to air pollution and ∼15% globally and ∼25% in the United States to emissions climate-changing gases. In the United States, the Corporate Average Fuel Economy (CAFE) standards for motor vehicles were significantly raised in 2012 for the first time in almost three decades. The standards now call for an average across manufacturers of 54.5 miles per gallon (mpg) for new passenger cars by 2025, or 163 grams per mile (g/mi) of greenhouse gas (GHG) emissions, and of 47 mpg (196 g/mi) by 2021. The light truck standards, which include minivans and sport utility vehicles, call for fuel economy levels of 31 mpg (287 g/mi) in 2021 and of 40 mpg (222 g/mi) in 2025. Given approximately equal expected splits between car and light truck sales, these estimates imply combined car and light truck fleet averages of ∼40 to 41 mpg (217 to 222 g/mi) in 2021 and 50 mpg (178 g/mi) in 2025. However, because automakers have strategies to reduce GHGs in parallel with fuel economy, the actual in-use fuel economy of the fleet is expected to be somewhat lower. In this review, several automobile technology advances are described and assessed for providing relatively cost-effective solutions to meet CAFE standards for fuel economy and GHG reductions through 2025. The key points this review makes are that (a) loopholes in the current regulations will allow automakers to meet CAFE program goals with relatively minor incremental improvements to current technologies through 2025 and that (b) going beyond the current program, past 2025 to higher fuel economy levels and lower GHG emissions approaching 100 g/mi, is likely to involve heavier use of newer technologies especially including drivetrain electrification.