befor asking for improvments it helps to know where you stand, so what kind of gas mileage do you get ride now? what kind of driving do you do and why?
the Corolla S is EPA rated at 22/31mpg where the base modle is rated at 28/37mpg acording to
http://www.fueleconomy.gov/ if you drive 15,000 miles a year you would save $480 a year, or in the time that you have owned it you would have saved over $2,000 just in gas, and as gas prices go up that savings will go up with it, now a normal toyota corrola with good low rolling resistance tires kept at the presure recomended for highway driving (4-5psi higher the what is labled on your door jam) and keeping your speed resonable you should be able to see 40mpg or better.