The charge-transport parameters in a series of naphthodithiophene (NDT) and anthradithiophene (ADT) derivatives are investigated by means of density functional theory and molecular dynamics calculations. In the case of unsubstituted and alkylated NDT and ADT crystals, the results point to small effective masses for the charge carriers along either essentially one dimension (π-stacks) or two dimensions (within the molecular layers), i.e., where large electronic couplings or band widths are present. Interestingly, diphenyl substitutions can lead to small effective masses for both holes and electrons along the three dimensions. This implies that one-, two-, or three-dimensional charge-transport mechanisms can be realized in the alkyl and phenyl NDT and ADT crystals. In particular, the smallest effective mass (and, as a result, the largest expected charge-carrier mobility) is obtained in the diphenyl NDT and ADT crystals along the direction perpendicular to the herringbone molecular layers. Our calculations also point to large nonlocal vibrational couplings along the π-stacks in the unsubstituted and dimethylated ADT crystals.