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Using a large sample of spiral galaxies for which 21 cm single-dish and/orlong-slit optical spectra are available, we make a detailed comparison betweenvarious estimates of rotational widths. Different optical width estimators areconsidered and their limitations discussed, with emphasis on biases associatedwith rotation curve properties (shape and extent) and disk central surfacebrightness. The best match with HI rotational velocities is obtained withPolyex widths, which are measured at the optical radius (encompassing a fixedfraction of the total light of the galaxy) from a model fit to the rotationcurve. In contrast with Polyex widths, optical rotational velocities measuredat 2.15 disk scale lengths r_d deviate from HI widths by an amount thatcorrelates with the central surface brightness of the disk. This bias occursbecause the rotation curves of galaxies are in general still rising at 2.15r_d, and the fraction of total mass contained within this radius decreases withincreasing disk surface brightness. Statistical corrections, parameterized bythe radial extent of the observed rotation curve, are provided to reduce Polyexand HI width measurements into a homogeneous system. This yields a singlerobust estimate of rotational velocity to be used for applications of diskscaling relations.

Rotational Widths for Use in the Tully-Fisher Relation. II. The Impact of Surface Brightness