Tilt that occurs during the lateral overgrowth of GaN over an SiO2 mask from (1 (1) over bar 0 0)(GaN)-oriented stripe openings is systematically measured using X-ray diffraction. This tilt is measured for a range of 'fill factors' (ratio of stripe opening width to stripe period) and for changes in growth temperature and input V/III ratio. It is found that changes in these parameters have a significant effect on the extent and distribution of tilt in the overgrown regions relative to GaN directly above the underlying 'seed' layer. Correlations between tilts and stripe morphologies as observed in scanning electron microscopy are made, and an explanation based on the 'local' V/III ratio around the growing stripes is proposed to account for these results. The aspect ratio of the stripes in cross section (which is affected by fill factor and growth conditions) is found to be directly related to the degree of wing tilt, suggesting that tilt may be affected by GaN wing/SiO2 interfacial forces.