It is generally thought that larger wheels are faster than small wheels. I'm not sure of the physics of this... larger wheels are going to have more linear and angular momentum since they are heavier than smaller wheels, both of which will hurt acceleration. Of course, once you're going fast, that momentum will tend to keep you going fast. The larger wheels also tend to give a smoother ride. I suppose that larger wheels with a greater ground-contacting surface could also allow a greater pushing force to be applied to them before the polyurethane surface fails, allowing a more forceful pushoff before starting to skid laterally. Larger diameter wheels meet the ground at a more shallow angle than do smaller wheels, possibly reducing energy lost in wheel deformation at the point of ground contact (larger wheels wouldn't need to flatten out as much to contact the ground). At the same linear velocity, the larger wheels will be turning less rapidly than smaller wheels, reducing the speed at which the bearings need to turn, thereby reducing frictional energy loss in the bearings. The Spiritblades had 72mm (diameter) wheels by default. It seemed like the 78mm wheels were faster anyway... Speedskating frames are typically equiped with 80mm wheels. 80mm wheels would not fit into the Spiritblades without rubbing on the upper surface of the boot.