2001 Grand Prix getting bad in-town mileage

Hi. My 2001 Grand Prix has 230,000 miles on it and still runs well. Lately I have noticed that my In-town gas mileage has been awful. If I drive strictly highway miles I get around 29 MPG. When it is a lot of in-town mileage, though, it has fallen from around a 25 MPG average to around 18 MPG. With high gas prices I would like to fix it, but not sure what it could be. There aren't any service lights on so no codes to check. In the last year I did have the catalytic converter replaced and the down flow O2 sensor. Other than that I haven't had to replace anything else.