Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous energy profile. This book presents current research data from across the globe in the study of fuel efficiency, including the study of a new CO2 capturing PGS that has a high-efficient NPGE by utilising waste heat from factories; government intervention to promote the development of increasingly resource productive and efficient systems of energy production and consumption in Japan; as well as improving fuel efficiency of compression ignition engines fuelled with vegetable oil.