The effect of a sheared toroidal equilibrium flow on the tearing mode stability index, Δ', is studied. A flow modified external kink equation for a single helicity mode is derived in a toroidal geometry. The corrections to Δ' arising from flow contributions are estimated on the basis of a boundary layer calculation. Toroidal shear flow is seen to have a destabilizing influence on the tearing mode in a manner similar to pure axial flows in a cylindrical geometry. However, the increase in Δ' as a function of increasing flow is seen to be smaller than the cylindrical case due to beneficial magnetic curvature effects. Shear flow associated stabilization, as observed in many recent experiments, might be arising from other physical effects such as toroidal mode couplings which are neglected in the present model.