To some extent, geometric volume can be allocated this way as well. Other component specifications must be established through a more complex understanding of how subsystem performance relates to overall product performance. For example, fuel efficiency is a relatively complex function of vehicle mass, rolling resistance, aerodynamic drag coefficient, frontal area, and engine efficiency. Establishing specifications for the body, tires, and engine requires a model of how these variables relate to overall fuel efficiency.
A comprehensive treatment of flowing down specifications for complex products is beyond the scope of this chapter, and in fact is a major focus of the field of systems engineering. We refer the reader to several useful books on this subject in the reference list.