We examine the relationship of seafloor roughness and gravity-derived crustal thickness to both spreading rate and inferred mantle temperature using statistical analysis of a multibeam bathymetry and gravity data compilation of the axis and flanks between 54°E and 67°E at the Southwest Indian Ridge (southwest Indian Ocean). Our findings indicate that root mean square values of abyssal hill heights increase from 220 ± 20 m to 300 ± 20 m along flow line corridors that transition a well-constrained full spreading rate change from slow (30 mm/yr) to ultra-slow (15 mm/yr). Mantle Bouguer gravity anomalies, however, indicate no significant change in inferred crustal thickness at the spreading rate transition. In the axis-parallel direction, roughness of both slow and ultra-slow seafloor increases from 54°E to 63°E while inferred crustal thickness and/or mantle temperature decrease. These findings have implications for the relationship between spreading rate and melt production: they suggest that mantle temperature at slow and ultra-slow ridges may play a more important role than spreading rate in determining seafloor morphology. The lack of evidence for significant crustal thinning accompanying a change from slow to ultra-slow spreading rate lends support to focused subaxial mantle upwelling models.