The improved fuel efficiency of new model automobiles, and regulatory trends in recent years show that low fuel consumption will continue to be promoted in the future. However, it is unrealistic to believe that fuel consumption will continue to perpetually decrease, even if hybrid vehicle systems are combined with internal combustion engines (ICE) that run on fossil fuels. Eventually, a "great wall" will be reached. There are two ways of breaking through this wall and much effort is still required by engineers before this can be accomplished.Past papers have examined how to minimize CO₂ emissions and target values for each technological field related to the reduction of fuel consumption. To supplement these reports, this paper breaks down these targets for each technological field and presents example target engineering requirements for each part of the car body.Engineering requirements related to fuel efficiency for each part of the car body are completely different from conventional requirements. The keys for achieving the targets are achieving so-called "efficient benefit sequences" (described below in Sections 3.2.4 and 3.3.3) and the functional sorting method at the body-in-white (BIW) engineering stage. An efficient benefit sequence refers to an improvement that does not stop at its original targeted effect, but that also produces a secondary beneficial effect. The functional sorting method is a simple and comprehensive way to indicate the mass efficiency of a BIW.