The miles of range mean nothing as that can vary wildly depending on various conditions and colder weather will definitely effect it. You need to do a proper capacity test:
1) Fully charge the vehicle
2) Drive in EV mode ONLY until the battery is depleted (no highway driving, avoid cabin heat as much as possible to keep the ICE from starting. If it does, it invalidates the test)
3) Once depleted and it reverts to hybrid operation, safely stop the car and shut it off. The trip summary will list the kwh used for the trip. This is the number we need.
5.5kwh is the brand new plug-in capacity from the factory. Anything above 4.0kwh should still be 'good'.