Bad gas mileage

2004 chevy impala, V6 3400 SFI. It has 55000 miles. Automatic trans.I have recently purchased this car 4 days ago it is still under warranty, and it is GMC certified. This car gets about 12 miles to the gallon in the city. At least from what the fuel gauge is telling me and I know that is not right. These cars are good for 27 to 35 to the gallon on the highway.I wouldn't think I would get 12 in the city. I think something is wrong can you tell me if this is normal or is there something I need to know. Thanks for any info. You can give me