My 2004 Grand Prix is getting really bad gas mileage. It's getting about 8 to 10 city and 27 highway. It goes down hill when we are in the city . We could have 27 mpg on the information screen and by the time we leave the city it's at 10 mpg. It might get back up to 16 mpg on the way home but not all the time. I have done a basic tune up on it. Also it has a new converter, map sensor, mas air flow sensor, fuel filter. The throttle body has been cleaned. No check engine light. Can't figure it out. Any help would be greatly appreciated.
