The standard for measuring fuel efficiency in the U.S. has been miles per gallon (mpg). However, the Environmental Protection Agency's (EPA) switch in rating fuel efficiency from miles per gallon to gallons per hundred miles with the 2013 model-year cars leads to interesting and relevant mathematics with real-world connections. By modeling fuel efficiency, mathematics teachers have an opportunity to correct mistaken notions while bringing real-world mathematics into the classroom. The author, Bartkovich, notes that this investigation will lead to model fuel efficiency with gallons per hundred miles (gphm), and the advantages found in this new model will illustrate why stickers for the 2013 model-year cars display gphm as well as mpg. The beauty of this topic is its high level of interest and relevance for students as they consider purchasing and owning their own car. In addition, environmental mathematics and the mathematics of sustainability will continue to be relevant issues for students throughout their adult lives. By bringing this material into the classroom, students are helped to understand mpg and gphm, and can learn some interesting mathematics along the way. [A bibliography is provided.]