2005 Cadillac STS mileage

Is this based on a physical MPG calculation over a few tanks, or on the trip computers calculation? You would have to check the fuel trim values with a scan tool, to determine if there is a cause for concern. The powertrain management system will give you an indication of a fault that is severe enough, with the check engine light. There can be circumstances where fuel is being added at idle, or at specific points of operation, that may not reach failure criteria. That is where the fuel trim values would be indicative.