The ratio of the transverse diffusion coefficient to the mobility, , has been experimentally determined for ions drifting in Ne and Ar gases, in the presence of a uniform electric field. Results at 303 K are reported for electric field-to-neutral gas number density ratio E/N range 5 - 600 Td, with an overall accuracy of %. The results were effectively corrected for longitudinal end effects present in the drift tube. In addition, Monte Carlo simulation (MCS) calculations were performed for the mobility and for transverse and longitudinal diffusion coefficients using respectively the Koutselos, Mason and Viehland (KMV) interaction potential and a modified Tang - Toennies (MTT) interaction potential as input. The accuracy of the MCS calculations is estimated to be within 1% for K, 2% for and 3% for . Comparisons of the MCS results with available experimental data give an indication of the validity and accuracy of the respective assumed interaction potential at different interatomic distances. The calculated results did not reproduce the experimental data very well, suggesting that both potentials should be modified.