Just to add my 2 cents worth here;
I have a '97 Prizm with the 1.8 liter engine and overdrive transmission. I have kept pretty good records with regards to fuel usage and I can say with a degree of accuracy what kind of mileage I average.
Over ten years, I have gotten an all-time low of 13 miles per gallon city driving in the dead of winter, (long warm ups, rush hour traffic congestion, drive-thru waiting lines, etc.) to an all time high of 41 miles per gallon, (with 3 people in the car, A/C on, highway driving @ approx 65- 70 mph).
Overall, city mileage for me is about 24- 26 MPG in the city, including frequent stop and starts and around 35- 38 MPG on the highway. The fuel economy sticker on the car when it was new estimated it at 27 city and 34 highway, so I figure that my numbers are both accurate and realistic.
Oh, and I always change my oil every 3K miles and keep my tires inflated to the max recommended pressure stated in the owners manual. I also follow the recommended service schedules as well.
Main things to remember, any acceleration from a dead stop, or idleing in traffic/drive-thru lanes will significantly lower your fuel economy numbers!
Help this helps a little!!!