If the car idles smoothly, accelerates and cruises reasonably well, able to down shift in a passing situation it would be difficult to diagnose without some serious investigation. The OBD II system will give access to a very good data stream that might show the cause of the poor mileage. Checking codes is not enough. MAP and O2 values would be prime suspects. And a partially restricted exhaust can lower manifold vacuum and cause the MAP to register a weak signal which the ECM would register as high demand for power and increase the fuel. If that were the case the catalyst would be quickly melted down though. If you feel like tackling it yourself find an OBD scan tool that reads the data stream and hook it up. It might pay for itself quickly.
Rod-Knox
17
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