Maybe a SCT tune?

My 98 Mark gets around 24.5 to 25 mpg on the road but only gets 16 to 16.7 around town. Now if I got a SCT tune (?) would that possibly help my mileage?....I've read different threads where folks get 28-30 on the road and 18-22 around town....if possible I'd like to get the same.....I don't know what an SCT tune does....does it change (modify) the engine management computer?.....can my engine be tuned for economy or performance?......I'm in the dark here folks and I could use some information from those in the know on SCT tunes.... and where would I go to get the car tuned?.......thanks in advance.....bruce