Firstly I'd say it is often difficult to get an accurate mpg figure from the indication on your fuel gauge. If you want a more accurate way of reading mpg then you need to fill your tank and then measure the distance driven until the next time you fill. The filling station measuring system (the petrol pump) is more accurate.

Next you need to make sure that your odometer / speedometer is as good as it says it is. Then you'll get a more accurate assessment of your mpg - although it will probably be an average for many types of situations. In Europe the sides of the motorways are often divided by set distance marker points - for example here in Holland every 100 meters - so you can count these points and compare that number with your odometer... Do you have that in the US too? (I've never noticed this sort of thing in the US before though)

If you still end up getting poor mpg then it will probably be driving style or tyre choice - as it looks like you've got a well serviced car there - sorry!