2011 mustang Jumps from 1st to 2nd gear if im not pressing the gas enough

[bsanch3]

Just bought a 2011 mustang and I noticed some problems after driving it home. When driving in traffic or driving slow in a parking lot it will jump from 1st to 2nd gear ( not smooth) but this only happens when I don't press the gas enough. Second thing is that my speedometer reads about 20 mph faster than I'm going. I was traveling at 50 mph but my speedometer said I was traveling 71 mph. Cars were passing me and my friend called to ask why I was going so slow and that is how I figured it out. When I come to a stop the speedometer goes back to zero so its not misplaced. Anyone have any suggestions of what could be doing this? I suspect the two things are related and I am not sure if the gear ratio was changed before I bought it.