The laws in each state are different. Particularly when it comes to motor assisted bikes.
Here in AZ, the limits are 48cc or less, and no faster than 20 MPH. If it is clocked over 20 MPH, or even if the cop SAID it was going over 20 MPH, it is considered a motor vehicle (moped) by the law/courts, and a moped requires license, registration,
and insurance... One of our members (Torques) spent a couple of days
in jail, and had over a thousand dollars in fines* (No license/registration/insurance, driving a motor vehicle with suspended license,) because the cop said that he was over twenty MPH (he had an earlier DUI and suspended license.) This is even
after he showed the cop his odometer maximum speed reading of 19.5 MPH. (The cop had a grudge against him from an earlier, failed attempted ticket, when he didn't know about the new MB law here.)
Ref
http://www.motoredbikes.com/showthread.php?t=6800
(* The fines were suspended, but, Torques got to pay nearly $400 for his lovely retreat in the city jail, plus paying for a lawyer...)