I drive pretty mellow most of the time, since I'm in rush hour traffic. I usually see around 21mpg on avg/30mpg on trips. This cold weather will affect the milage quite a bit, but not to the point you are at. Make sure the basics are covered; clean air filter, pcv, oil change. 02's should be replaced after 50K miles, if you do this the best hookup is Justin from Casper's. They carry original AC Delco's pretty cheap. The injectors can be easily cleaned with a can of Chevron Techron additive. It's good preventative maintanance. Try these first before doing anything expensive.