This study investigates the effects of the channel and rib dimensions of the flow field on the performance of polymer electrolyte fuel cells (PEFCs). The development of a channel/rib geometry, where dimensions are adapted to local conditions along the reaction pathway, is described. The optimal channel/rib sizes at a given position along the channel is based on the results of local current density and membrane resistance measurements on the submillimeter scale. The performance of single cells shows that under certain operating conditions, a properly adapted channel/rib dimensioning can lead to an improvement of the cell performance compared to designs with constant channel and rib dimensions. An optimized adapted geometry has narrow channels in the inlet region to prevent membrane drying and narrow ribs toward the outlet to reduce transport losses in the porous gas diffusion layer at conditions when the reactants are saturated and depleted. The improvement of integral cell performance is highest with low cathodic stoichiometries.