I own a 2003 Kia Optima, there has been a strong gasoline odor coming from the rear of my vehicle. There are no leaks and the gas mileage hasn't been decreasing much. Also, not sure if this has anything to do with it, but my rear passenger tire has been squealing on acceleration but quits when I tap on the brakes.
This is a common problem in kia vehicles
The gas tank has a vent to the atmosphere. When its hot out some of the gas will vaporize in the tank as it gets hot. The vent is there to relieve this vapor so you dont explode the tank. I believe that this relieved vapor is what you're smelling.