This paper describes a preliminary study of the role of computational fluid dynamics in analyzing the aerodynamic characteristics of a heavy-duty truck body. Among truck related aerodynamic problems, we selected the soil problem on the vehicle side surfaces as the analysis subject. Because of computer capacity limitations, a half-truck-cab model with a tire and mud guard are used and created by using the multi-block transformation technique. The flow around the cab is simulated by directly integrating the Navier-Stokes equations, approximated by finite-difference equations. Calculated results on the flaw structures around the vehicle body surface where it becomes dirty under wet weather conditions provide some useful information in the search for understanding of soil problems.