Well, difference in tires could be playing a factor.
Also try cleaning your MAF. That might help. As well as the throttle body. Changing out your o2 sensors could help. Make sure your flex pipe isn't leaking or anything.
^Those are all issues I've run into that have killed MPG's. But other than that, every car is different. Some will exceed expectations while others fail to reach the minimum. Most likely, that's the problem imo.
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