My Ruckus goes about 45 but it does take a bit of time to get there.
As long as you keep on the 40mph or less streets you´ll be fine. If not there are plenty of go-fast modifications you can get to pep it up (www.totalruckus.com for info). Or you can opt for the Big Ruckus. It´s 150cc as opposed to the Lil Ruckus 50cc engine. Accelerates much faster and has the same styling.