The friction coefficient was measured for developed flow of drag-reducing polymer solutions in tubes of 2, 5, 10, 20 and 52 mm i Our results were processed along with other authors＇ data in terms of different parameters in order to investigate the possibility of developing a simple empirical method for the prediction of the pipe diameter effect on friction. We found that the drag-reduction coefficient (DR), if expressed as a function of the fluid bulk velocity (V), becomes independent of the tube diameter in the subcritical region (i.e. without fluid degradation), with the deviations being smaller than about 5% for all the diameters and velocities covered. This correlation proved to be not only better than similar procedures based on friction velocity, but also more convenient and physically more meaningful. It was also found that the logarithmic layer shift in the 3-layers velocity profile is also better correlated with the bulk velocity than with the friction velocity. Finally, existing models for drag-reduction involving non-dimensional correlations between the integral flow parameters and the fluid properties were also reevaluated in light of these findings.