In a number of recent studies, information about the structure of large polyatomic ions has been deduced from gas phase ion mobility measurements by comparing mobilities measured in helium to those estimated for assumed geometries using a hard sphere projection approximation. To examine the validity of this approach, we have compared mobilities calculated using the hard sphere projection approximation for a range of fullerenes (C-20-C-240) to those determined from trajectory calculations with a more realistic He-fullerene potential. The He-fullerene potential we have employed, a sum of two-body 6-12 interactions plus a sum of ion-induced dipole interactions, was calibrated using the measured mobility of C-60(+) in helium over an 80-380 K temperature range, For the systems studied, the long-range interactions between the ion and buffer gas have a small, less than 10%, effect on tile calculated mobility at room temperature. However, the effects are not insignificant, and in many cases it will be necessary to consider the long-range interactions if the correct structural assignments are to be made from measured ion mobilities.