Bought a used 2011 GT at a dealer last month with 52000 miles on it. Had a whining/howling noise on deceleration at 40 mph and I noticed thick black oily residue all over the passenger side axle housing. Took it to the dealer last week, they found it was leaking rear end fluid and that the ring and pinion were shot and that they were 3:73 gears that they assume the last owner had installed. The dealer replaced the ring and pinion with stock gears which they said were the 3:15. Now, we have speedometer issues...I'm between 5 -10 mph off depending on speed. Took it back to the dealer this morning, the tech said when he went to flash the computer that is was already current and that he will have to get another technician to do a side by side test drive with another mustang gt and compare mph numbers and report it to Ford so they can look in to this further. Does this sound right? Any thoughts are appreciated.