Are you only using the on-board computer readout for your mileage? I would track it yourself for a full tank and compare (start with full tank, mileage at zero, then after driving most the tank fill it back up and divide the miles by the gallons used). I've noticed my computer consistently reads 2-3 mpg lower than what I am actually getting. Just a thought.

 

5 freeway is usually the worst. i get like 28 while driving on it. lol