You probably are going faster because the tire will conform to road irregularities better and do less bouncing over them. If you ride glass smooth roads it may be a non issue but I know my roads aren't like that. They also corner more stable because you have more rubber on the road. I haven't ridden a 23 in probably 8 years now. No measurable difference in average mph, just a smoother ride. Heck, on my bike with 32's I still have about the same average mph.