Not quite sure on my part i recently went from 25mpg to around 21. Kinda odd because the car is mechanicly sound and the weather is warming up. Checked the tires/pressure as well as the filter. I also only use premium gas. I can only think its the spark plugs left to check but i couldnt see anything wrong with a car only at 8.5k miles.

 

Also, don't forget that a tire's rolling resistance goes up as the temperature drops, and vice versa. If the vehicle is driven mostly in the city, for short distances, the tires may never get up to optimal temperature. As a result gas mileage will reduce.

 

Altho i dont own a S yet but i would say if your driving over 60, the wind is hitting your car more strongly and in result you end up using more gas trying to maintain the speed your at 60+ i just did a 300 mile trip and my car usually bangs out only 13/14 mpg on city lucky/ but on my long trip i used cruise at 60 and i banged out 21 mpg.

 

Maybe it's because I drove a lifted jeep prior to the S, but I think my MPG is pretty damn good even if it is only 20 mpg, lol.

Changing your plugs could be a good way to boost that. I did mine today and it made my car a lot smoother and you can feel that the old plugs (which looked horrid) were hurting performance a lot. I usually do about 25 mpg with city/hwy.