Well, considering my 08 GP is rated for 18 mpg city, 14 mpg isn’t overly bad. That being said I would say someone messed up the spark plug installation. Maybe it is a disconnected wire or broken plug possibly. Also, if your car has over a 100k, the O2 sensor is on borrowed time, getting good mpg with an in accurate O2 sensor is nearly impossible. Keeping in mind bad mpg can be caused by a dozen things. Good luck.