For 2008 the government mandated a change in testing procedures to more closely reflect actual mileage. That is why 2008 vehicles show a lower mileage rating on the stickers over the exactly identical vehicle from 2007 (or in this case 2005).

A couple things you can do to improve your gas mileage would be a CAI & tuner combo or just a tune. Stock the car runs rich for safety so by leaning it out a little you use slightly less fuel but you have to hold back from the urge to floor it. Also, you can have nitrogen put in the tires instead of air from a compressor. Nitrogen weighs less so it decreases your unsprung weight meaning a small increase in mileage. Plus, nitrogen doesn’t bleed off nearly as quickly as air so you have fewer issues with your tires being underinflated. You can also buy wheels that are lighter than your stock wheels to increase the unsprung weight savings. But the best way to save fuel is to learn to drive in a fuel saving manor. My 06 Mustang GT has seen a best of 26 mpg on the highway yet by recently changing my driving style ($4/gallon keeps my foot out of it) I have increased my around town driving from 18. 6 mpg to 20. 1.