I have an '05 impala ss that has been averaging 25-26 mpg on the highway. I usually keep the speed around 70 mph. I do live in Wisconsin, so I may be using winter blended fuel (I never heard of winter gas though). The car is a gm ceritified used vehicle which I bought back in november and now has about 38000 miles. I did put a new t-stat in (engine runs now runs about 185*). I would think plugs would have been changed before I bought it, so should I go for the wires or something else?
While Ben is right, winter gas and the cold will make your mpgs a bit lousy. Your car is supercharged ( I think it is), it requires premium gas and if you use premium gas which it does recommend, the car will get better mpgs from it and essentially also prevent knock whihc can damage the motor.