2000 Nissan Maxima Gas mileage dropped significantly

I purchased this car about two weeks ago and last week I took my car to a mechanic because I heard a slight thumping noise coming from one of the rear tires when I would drive at low speeds (<20 m/hr) on smooth roads. When the mechanic put the car on the lift and spun the driver side rear tire by hand he could hear it. He at this time replaced the calipers and the rotors. As I left the shop the alternator went south on me and I returned and had it replaced. However, now I hear a fluttering noise which I believe is coming from the same side and tire as where the brake system was replaced. It also appears that my gas mileage dropped significantly. I was getting about 70 - 80 miles to a 1/4 tank now it is at about 30 - 40 miles. Any ideas on what this could be?