Measured ultimate tensile strengths in unidirectional fibre-reinforced composite materials have been observed to deviate from the linear predictions of the classical rule-of-mixtures relationship. The physical factors responsible are fibre-fibre interaction, inhomogeneous fibre distribution in the matrix and fibre misorientation to the loading direction. A recent modification to the classic rule-of-mixtures to account for fibre-fibre interaction has already resulted in good agreement between measured and predicted values of ultimate tensile strengths at high fibre volume fractions for Kevlar/epoxy composites. Additional modifications to the rule of mixtures to incorporate both fibre misorientation and inhomogeneous spread have been presented here. These modifications result in greater agreement between measured and predicted ultimate tensile strengths at low fibre volume fractions while retaining the accuracy of prediction at higher fibre volume fractions. Good agreement between measured and predicted values of inhomogeneous fibre spread were obtained at high fibre volume fractions. Furthermore, these additions to the classic rule-of-mixtures can be used to gauge the extent of each of the physical factors responsible for ultimate tensile strength reduction in unidirectional composite materials.