The standard three class electric bicycle definition was enshrined into ARS in 2018. There is no explicit speed limit for electric bicycles; but unfortunately, the mention of 20/28mph in the definitions has led some to mistakenly believe there is, in effect, and e-bike speed limit.
This is not the case. Where an electric bicycle meets a class definition, it may be operated at any speed; so long as it meets Arizona’s “basic speed rule” which typically boils down to no faster than reasonable and prudent. You are probably aware that bicycles routinely go faster than 20 or 28mph! Electric bicycles are bicycles.
Here is Arizona’s basic speed rule, it applies to bicyclists (including e-bicyclists) because they are subject to the laws applying to the driver of a vehicle §28-812.
28-701. Reasonable and prudent speed; prima facie evidence; exceptions A. A person shall not drive a vehicle on a highway at a speed greater than is reasonable and prudent under the circumstances, conditions and actual and potential hazards then existing...
Modifying an electric bicycle so that it no longer meets any class, or buying an e-bike that is out-of-class to begin with makes the, er, ‘device’ no longer able to be operated on Arizona roads as an electric bicycle… The only legal way around that is to claim it’s a motorcycle and somehow get it registered (if that’s even possible?) and insured and have a motorcycle-endorsed driver’s license. See the story about Motorized Bicycles (which despite also being bicycles with helper motors do, by the way, have an operating speed limit of below 20mph) for many references.
Usual disclaimers apply: I am not a lawyer. The above discussion applies only to Arizona state law. Local laws may vary.