I have gotten as high as 35 mpg on my 99 mistique on mostly highway, it is a bit lower in winter with headlights, wipers and defroster on, around 27city this time of year. mine is a manual tranny, but many things can factor in.
besides maintenence, check tire pressure weekly, also driving habits like jack rabbit starts can have a negative effect, also in case of a manual gear box, run it in the highest gear you can for the driving conditions. for the most part, gas mileage is in direct porportion to the weight of the right foot.
