1998 Chevy Cavalier mpg performance

I have done everything from a simple tune up to minor modifications to try and gain gas mileage. Now I will tell you that I do have a lead foot but 75% of the time im driving normally. The car is estimated at 25mpg and a little more on the highway but im getting around 15 in town. I cant find a software system to load into my laptop that I can connect to the computer in the car to find out why im not getting the performance I expect. What can I do and where can I find a software system to help me get the performance I expect