Hey fellow Law Enforcers,
Might be better to keep a good structure inside your department besides just your police ranks. In real life ranks are often given to people in charge of certain Divisions, Precincts or Specialisations. For example often a person in charge of a Office is a Deputy Chief of Police, someone incharge of a precinct is a Captain, persons in charge of a Division are Lieutenant/Sergeants and persons in charge of Patrols or a watch is a sergeant.
This being said there is still a need of a hierarchy between the ranks. Often you start out with the Commissioner who will be in charge of your police forces and executes a observing and controlling role. Every city will have their Chief and Deputy chiefs to control their city with imput of the Commissioner. You should not look at ranks for your police structure, you should look at departments.
I made a small chart to give an example of how to make a structured police department.
http://i.imgur.com/EpvsIqM.png
It's also entirely possible for a division to be led by a sergeant or Precinct by a lieutenant however you shouldn't have 1 captain for every 2 lieutenants. It should depend on their quality not their quantity.
[POLICE] Organisation & Structure
Link added for more information on this.