I have a feeling you are trying to see where you would fit in to the heirarchy...is this the case? If it is...go do some group rides to see where you fall...or just sign up for a race.

Categories are not based on speed. You could have 5s that could cover your ideal flat course faster than 3s. Racing is all about maneuvering yourself to the finish line ahead of everyone else through strategy, pack riding techniques, sprinting, etc.