We study the effects of a differential rotation upon the determination of the v sin i parameter. The effects are studied for several values of the ratio t = kinetic energy/gravitational potential energy, which include energy ratios higher than permitted for critical rigid rotation and using an internal conservative rotation law that allows for a latitudinal differential rotation in the stellar surface. Two effects are outstanding: when differential rotation is dependent on the stellar latitude the v sin i parameter does not necessarily correspond to the equatorial rotation velocity; the line width is a double valued function of v sin i and it is dependent on t and the aspect angle i.