Romney has repeatedly taken issue with the updated CAFE mandate that was agreed to during the first Obama term. They call for a 54.5 miles per gallon fleetwide fuel economy level by 2025 (a real-world equivalent of about 40 miles per gallon), roughly a 30-percent increase from current levels. The standards would make new vehicles too expensive for many consumers, the National Automotive Dealers Association (NADA) has said. In late August, the federal government finalized the CAFE standards for model year 2017-2025. The 54.5 miles per gallon mandate was first proposed in July 2011.
Meanwhile, Romney, which hasn't detailed any alternative proposal to the CAFE standards, has suggested the federal government invest more in research and development and fewer government investments in automakers, whether they be traditional OEMs like Ford and Nissan or alt-fuel specialists like Tesla and Fisker. For more on the energy policies of Romney and Obama, read this.