Speed really has nothing to do with the size of your wheel. It's largely a matter of fitness and enough gear inches. With that said, I know I can sprint faster than 30 mph on a flat for a short distance. I couldn't maintain it for a long period of time, but that's why they call it a sprint. I will tell you one thing-my twenty easily weighs 29lbs so if iknow I'm going out on a fast group ride, I would sooner grab my 18lb road bike.