Underbody diffusers are used on racing cars to generate large downforce that will permit them to achieve reduced lap times through aerodynamically-enhanced traction. Both the configuration of these cars and their underbody flows are complex, so the design of optimum underbody geometries is a formidable task. The objective of the present study is to generate data and understanding that will facilitate design through knowledge of the relevant physics and the application of a numerical analysis that provides generalised design guidance.The approach taken is one that is traditional in the study of complex problems: to identify a less-complex but still relevant sub-problem that has the key elements and flow physics of the main one, and study it to generate a first phase of cause-and-effect relationships. In addition to having immediate utility, it can serve as the foundation upon which future research activity can be built.The result of the present study is an analytical model that will facilitate the selection of optimum length and area ratio for the underbody diffusers of flat-bottomed racing cars. While not a universally-applicable design tool, the guidelines developed should reduce the effort required to develop underbody configurations that produce large downforce.