The present study deals with the effects of wall geometry on fluid flow and heat transfer in a circular tube. Rectangular-rib structural wall roughness is considered. This roughness is characterized by rib height, width and pitch. A detailed parametric investigation of the effect of rib width-to-height ratio on the friction coefficient is performed for different rib pitches. This ratio varies from 0.1 to 6.0. Various rib pitches are considered, from those corresponding to a smooth wall, and up to the pitch-to-rib-height ratio of 16. The pipe diameter and the flow Reynolds number serve as additional parameters. Numerical simulations are performed using a modified kepsilon turbulence model, which makes possible the prediction of flow separation, reattachment and adverse pressure gradients. The model is validated by comparing its results with experimental results reported in the literature. The results of the present study indicate that for the given pipe diameter and flow Reynolds number, the pitch that corresponds to maximal flow resistance depends on the rib width-to-height ratio. It appears that when the width-to-height ratio decreases, this optimal pitch decreases.