The other dudes are right + is there any excess weight your lugging around in the trunk. Every 100 lbs can take away a 1/2 mpg. Also, are you going the posted speed limit? Most cars are geared to get their best mpg's while going 60 mph. Any faster than that & the engine has to really push against wind resistance. The Gran Prix is a fairly aerodynamic car, but it's a heavy one.